The slim jim I put up for APRS was hanging right next to the feed line of the higher quarter-wave antenna I use for local analog traffic. The feed line was de-tuning the slim jim, so reception of APRS traffic was almost non-existent. I could swap the antennas and the higher one worked much better, and the lower slim jim was greatly degraded for other FM traffic. Usually the BTech is on the lower slim jim, and the Quansheng is scanning on the higher quarter-wave.
To correct the problem, I pulled the 2 antennas up on separate ropes and separated them by a couple feet. The quarter-wave is still mounted higher than the slim jim.