I would be curious to know if this problem is mostly rcving portable TX. If it is, your problem may not actually be a problem but you are dealing with plain old portable talk back issues. In that case, your CC TX ERP may be too high.
I don't know what your site antenna configuration or repeater power out you use. Consider the extreme case of using a 100 watt repeater on a single antenna with duplexer. Ain't no way a 5 watt portable, analog or any digital, can talk back as far as it can hear the 100 watt rptr. You can put all your rcvrs on 1 top mounted, hi gain antenna with filtering and pre-amps to improve performance, but ideal antenna systems are not always practical in the real world. Your symptoms indicate you can talk into the CC but not into the TC. Keep in mind that the data exchanged on the CC is more tolerant of high BER and low signal level than what is required to decode the voice data stream.
Having gangbuster repeater TX where portables can hear traffic out in the boon docks or in buildings eventhough they can't talk back might be OK for single site service but that is not what you want for multi-site. Subscriber units monitor the CC until the BER or signal level drops below a certain level, then they will look for a better site. It can be frustrating when your radio stays locked onto a site that you can't talk back to when you've driven well into the next site. The "FAILED" indication means that you can't even talk back to the CC.
Program some portables that you can disable the auto-roam and manually lock the radios on each site and test your talk back range. Normally a radio won't roam until the signal goes down to 1 bar. Find some locations where the radio is at a steady 1 bar and test your talk back. You can diddle with the roam threshold levels of the portable but we found that the better solution is to crank the CC power down in 3dB steps or half the power and field test the radios again.
I confess that we used to run high ERP channels with 100 watts because that is what we are licensed for and we would roll the CC over every night... until we added a bunch of portables that had the same complaint that you described. We now lock control on ch 1 and field tested each site for portable talk back balance. CC Settings vary from 20 to 60 watts. The lower power does not seam to effect the range of mobiles but it did cure our poor & no talkback complaints with portables, and improved our roaming function in overlap areas. We run our TC's at 90 watts because we can and it assures a good voice data path to the units.