I have a .ps1 file in which I want to define custom functions.
Imagine the file is called MyFunctions.ps1, and the content is as follows:
Write-Host
You certainly can define functions in script files (I then tend to load them through my Powershell profile on load).
First you need to check to make sure the function is loaded by running:
ls function:\ | where { $_.Name -eq "A1" }
And check that it appears in the list (should be a list of 1!), then let us know what output you get!