Alec

I agree with the previous posts that the most probable culprit is that the RT15 radios are using CTCSS or DCS Codes (see page 6 of your RT15 manual - I found it on the Retevis website here: Retevis RT15 Mini Lightweight FRS for Business Radio, usb charging, license free walkie talkie). Page 1 of that RT15 manual shows a diagram of the radio and according to that diagram the radio has a "Monitor" button on the side just below the PTT key. If you long press that button, it should, temporarily bypass any pre-programmed CTCSS/DCS tone code squelch. So, if you have all radios on the same channel (avoid channels 8-14 as your base station probably will not transmit on those channels) then long press that Monitor button on the RT15 while you transmit on the Midland GMRS base station and you hear the base station radio transmission come thru the speaker on the RT15 you will have pretty much confirmed that the issue is that your RT15 FRS radios have factory pre-programmed CTCSS or DCS Codes enabled. To solve this issue, you will need to reprogram these RT15 FRS radios to remove the use of CTCSS/DCS tone codes. As has been suggested by @AdmiralCochrane, you could also program your Midland 105 base radio to use the same CTCSS/DCS codes as the RT15 are using, but personally, I would not recommend that approach as doing so will limit your ability to communicate with others. Hope that helps.

Sorry, I should have been more specific. It's a 2018 GMC Sierra

From what I've observed magnet mounts seem to be rated in terms of "pounds of pull strength" which appears to be a rating of how many pounds of pressure or pull is needed to remove the magnet. Again, just casual observations on the various magnet mounts offered for sale via Amazon suggests that the larger the magnet mount diameter the more pull strength it tends to have. I would guess that getting a larger diameter magnet mount or one with more pull strength (or a smaller antenna with significantly less wind resistance) might allow you to overcome this issue of the mount slipping. FWIW, I've a GMC Sierra and use a magnet mount with a 32" whip, with no issues. But hey, I'm not a certified magnetologist, so your results may vary.