In the Play developer console it says:
Your APK does not seem to be designed for tablets
But I have added layouts to layout
It seems all to be about screenshots! I have quoted this from google play listing store section in google play console 24/09/2015 :
Screenshots: *
Default – Arabic – ar JPEG or 24-bit PNG (no alpha). Min length for any side: 320px. Max length for any side: 3840px.
At least 2 screenshots are required overall. Max 8 screenshots per type. Drag to reorder or to move between types.
For your app to be showcased in the 'Designed for tablets' list in the Play Store, you need to upload at least one 7-inch and one 10-inch screenshot. If you previously uploaded screenshots, make sure to move them into the right area below.
I uploaded new screenshots, the problem was solved!
Actually depending on the screenshots is more logical than depending on analysing the APK. Since some apps has not lots of contents to show!
Happy Androiding!
Add in file build.grandle
compile ‘com.android.support:appcompat-v7:19.+’
where 19 - it's compileSdkVersion
Same in my case like @Wytas and @blackdigger, images were problem too. So I generate the images from http://developer.android.com/distribute/promote/device-art.html then warning message gone.
I don't believe that this is a completely automated process. In other words, I think there is someone in Google looking at screen shots from your app making a qualitative judgement (probably an intern). This is my reasoning for arriving at that conclusion:
I have three versions of my app (Irish, UK and USA). They are basically the same with different text. The Irish version was released first and initially I encountered that message. I had implemented the manifest (like yours) and had one or two xml files in layout-sw600dp and layout-sw600dp-land and I couldn't see the cause of the problem. I uploaded a new apk with new screen shots (no xml or manifest changes) and the problem went away. Here is the current status of the Irish app:
.
Next I implemented the UK version. It displayed "when you did it" as 12th June, the date the APK was uploaded. So at least there is a semblance of consistency in the review process.
Finally - the US version. To reiterate, this is basically the same app with the same xml as the Irish and UK versions and this is my current message:

Now it is quite possible that Google have written some code to look for xmls for large tablets. But if they had, I think my other APKs would be showing that error as well. Therefore, I think that the review process is partly manual.
So my conclusion - a quantitative approach may help (a few more xmls in sw600dp and sw720dp) but don't neglect the qualitative stuff (choose screen shots that don't display empty space !).
Update: re. the "Designed for Phones" message - December 2013
Since I wrote the above Google have introduced the dreaded "Designed for Phones" tagline and "Design your app for tablets" optimisation tips. This appears to be largely based on an automated scan of the apk when you upload it (probably the xml), so the process now seems to be more automated than was previously the case. Here are some points I have discovered which may help others:
1) I was unhappy that Google had designated one of my apps as "designed for phones" and I emailed them and they then removed this designation.
2) On a different app, I was getting the "design your app for 7" tablets" optimisation tip. I added one fragment to my tablet xml in sw600dp like this:
<FrameLayout
android:id="@+id/SideTab1"
android:name="SideTab1"
android:layout_width="300dp"
android:layout_height="fill_parent"
android:layout_marginLeft="20dp"
/>
and this did not remove the optimisation tip.
3)However when I made the same change to a second xml in sw600dp then the "design your app for 7" tablets" optimisation tip was replaced by "Your layout should make use of the available space on 10-inch tablets". More importantly, the "designed for phones" designation has now disappeared.
4) Obviously my next step is to make similiar changes in sw720dp.
Things seem to have changed again now (e.g. Google seem to now have removed the "designed for phones" designation in the country in which I am currently residing, and they seem to have relaxed some of their "rules" regarding the "design your app for tablets" message). Apparently, I am now doing "pretty well", which is reassuring news !
I think you may have problem with layout folders layout-sw600dp, layout-sw600dp-land, layout-sw720dp and layout-sw720dp-land
Try layout folders like layout-xlarge, layout-xlarge-land and put your dimens at values-sw600dp and values-sw720dp
And also have a look at the below link
http://developer.android.com/distribute/googleplay/quality/tablet.html
I read somewhere on the play store page that it needs some screenshots for 7" and 10" tablets in order to be featured as "for tablets", whether there is a specific layout for it or not.