are ytou getting any error message or indication as to why UCASE is failing. If you are not gettign any error messages then I would expect it to be something that is happening in the code somewhere.
To satisfy yourself that UCASE works: As a quick check open a form in design mode, view the code window. In the intermediate panel of the code window (the lower panel) type ?ucase("sometext"). Assuming that eturns "SOMETEXT" then it would suggest that the error lies elsewhere, ie in the form or table design.
There could be some reformatting hence ansentrys suggestion of coercing the value to be upper case by setting the table format to ">". Indeed it could be that your table, and of form my be set to "<" (Lower case).
The UCase function is OK on my developer PC. The problem occurs only on RT versions of my application on certain PC's
Could the DLL be the problem? Is there a way to "force" loading the required DLL (VBE6.DLL)
Check the references list in your VB code module on the machines that are malfunctioning.
I just tried to install an Access Data Project at a client and got an error message stating the "LEFT()" was an unknown function! What the? Turned out that some code references were missing when Access was installed on the PC.
If it's not practically useful, then it's practically useless.