Acrobat X keeps asking for the serial number

We have a couple users who's Mac's are asking for the Acrobat Serial Number several times a day. The license is part of a volume license (Not subscription) from CS6 - we have about 45 users working from this 50 user license. Two users have this issue. Anyone know a solution?  Is there a corrupt file?  They enter the serial number and it accepts it and opens just fine, but later it will ask again.  Mac OS X 10.9.5 and 10.11.6 Acrobat Pro 10.1.9

TIA!