I use a magnetic mount to attach my relatively small VHF/UHF antennas onto the car for my daily activity on the commute to and from the station. I tend to need a new magmount every 12-15 months as repeated opening and closing of the hatchback eventually takes its toll on the coax!
The last but one magmount I had worked really well for the usual period on all the VHF/UHF bands I use from the car. It finally gave up and I replaced it, with one from the same retailer, with ostensibly the same spec. It certainly worked, but particularly on 70cms, the performance was considerably worse. Not catastrophically worse, more like 'conditions aren't very good today' worse - so I just put up with it.
I had the opportunity this morning to try a different magmount - a slightly more expensive model (the original one I was using retail at around £16 and the newer one at about £25). On 433MHz the difference was enormous! GB3TD which had become unusable at the far end of my commute near Didcot, was just fine! I don't think it was just conditions. but it will be interesting to see how the new mount affects the other bands.
So now I'm wondering what was 'wrong' with the old magmount. It's not as if it really didn't work, but perhaps it was just very lossy. Either way, I'm glad I tried the different mount. Your mileage may vary, but you may be well advised to spend slightly more on your next magmount!