Top of arms is fine however where exactly on the arms ? Outside/inside/front ?
Sensor failure through the night can be attributed to compression applied to the sensor, I don't use the Guardian sensors - have been using Dexcom for over 5 years now, however when asleep, sleeping on it can depress the sensor causing a compression low which gives a false reading and i'm guessing with the Guardian can give a failure alert. Placing sensors on the bingo wing can help prevent this, I place my sensors on the inside of the arm to avoid it - also with using sensors it's good to place them around 12 hours before they are actually used to allow them to bed-in and helps avoid the erratic first 12 hours, some people call it marinating..!
It's easy to give up on tech if it doesn't comply immediately - however a little patience is required to adjust and fine tune to get the best results, stick with it though.