Initially, it does sound a bit like a priority look-back. Make sure that's not the problem first.
Lower CTCSS codes seem to work better in the 100-series, but sometimes you can't change the CTCSS system-wide.
The 100-series comes out of the factory with a fairly tight CTCSS decode criteria. I have had problems with the 121S installed in voting receivers being slow to initially decode and open. Not a big problem in simplex, but in JPS voting systems the receiver selection takes place in the first 1/10 of a second. A F121S that takes 1/2 second to decode CTCSS is missed on the initial vote.
Try this: read the radio with the cloning software and open "Maker Reserve". I won't say how to do that, but try Google. No warranty and all disclaimers apply!
SAVE A SAFE COPY!! Make sure you don't overwrite it later. Maker reserve writes radio-specific information to the radio that is not sent during a normal clone write. Your safe copy may let you restore a screwed up radio.
Change "CTCSS; Decode Attack" down from the default 140 . You are loosening up the "strictness" of the CTCSS, effectively opening up the squelch. Try 120. Minor changes will make major differences. Don't go below 100 or you may defeat CTCSS decode entirely. Don't change anything else!
Write to the radio and shut down the software completely to avoid screwing up. Do a trial an error to find the highest number that will work. Be VERY careful in maker reserve or you will brick the radio- that's why its hidden.
See it this helps.