The problem with multi-channel USB audio interfaces in general is that they're designed for a music recording environment, with close-mic'd instruments and vocals producing plenty of level out of the mics. Compared to mixers/preamps designed for speech/drama recording, they typically have 10 or 20dB less gain (eg. max sensitivity of around -50dBu rather than going to down to -60dBu or -70dBu as a good TV studio mixer would do). The UR44 has a maximum sensitivity of -52dBu.

You may be able to turn up the output level on your radio-mic receiver packs, which would obviously help, but there's probably nothing fundamentally "wrong" with the mics, more a case of using other equipment designed for a slightly different purpose. I've not checked the specs on all the other USB audio devices out there, but the ones I have checked have all been similar, although you may find one somewhere that has a higher sensitivity than -52dBu. Other people may have different sound sources with different output levels, so it's always worth checking the specs before you buy, whatever experiences others have reported.