Set field works fine for me and I've used it hundreds of times in FileMaker 10 and 11.
Perhaps there's a problem with the file. Create a simple two field test file with one field set to be a global field and test that file to see if it works consistently. If it does, you may want to run a recover on your file to see if it's damaged. (Sometimes a damaged file will appear to work correctly with one version of FileMaker and not another, so this may explain why it works with v9.)
Best practice is not to use a recovered file if you have any alternative such as importing your current data from the recovered file into a clone of an undamaged back up copy.
My co-worked found the following post in this forum "This record cannot be modified in this window (Thanks PhilModJunk for the answer to that post!)
That's the answer to my problem too. The edit screen is a pop-up window. I had an Open Record step in the Script that opened the edit screen from the main screen. The Open Record step executed in the Main screen. I need to commit that record in the Main screen before I can use the Set Field step to change any field value of the same record from the edit screen.
I deleted the Open Record step. Now the scripts are working in v11.