If you are using v1.9.6, see the corresponding Readme (sec. Bugfixes, 1st entry, https://github.com/Rdatatable/data.table):
if (TRUE) DT[,LHS:=RHS] no longer prints, #869 and #1122. Tests added. To get this to work we've had to live with one downside: if a := is used inside a function with no DT[] before the end of the function, then the next time DT or print(DT) is typed at the prompt, nothing will be printed. A repeated DT or print(DT) will print. To avoid this: include a DT[] after the last := in your function. If that is not possible (e.g., it's not a function you can change) then DT[] at the prompt is guaranteed to print. As before, adding an extra [] on the end of a := query is a recommended idiom to update and then print; e.g. > DT[,foo:=3L][]. Thanks to Jureiss and Jan Gorecki for reporting.
Thus: Does calling DT[] after your function call help?
