We also had this problem a few months ago.
I found the following on the microsoft forum:
Open Filemaker and perform the steps and see if it is working.
a. Right click on the taskbar.
b. Select “Task Manager”.
c. Click on “Application” tab.
d. Right click on the “Filemaker” program.
e. First select the option “Maximize” and check if it is working.
f. If not, select the option “Bring to Front”.
Once you are done with this, close the program and re-open to see whether it is fixed or not.
Let me know if it works
Thank you for the response.
I never got to trying your method...before doing that I did a full uninstall and a reboot.
I installed again and this time it worked just fine.
So not going to worry too much at this point but I will keep your reply in hand for future.
It maybe registry corruption. Uninstalling FM clear the settings of window position/size.