You will need to step through and see where you are locking.
If you are call an RPG program with that file as an UPDATE file and you have a conditional update, You do not release the record until the next read.
So you could have it locked before you call to the SQLRPG.
Use debug an stop before the call to the SQLRPG and then look at your job locks and see if you have a record lock.
If not, the we need to see the logic of your SQLRPG program.