You have to put BPi M3 as near as possible to the AP. Try to reduce distance to 1m or maybe 50cm! Everything above 3m is known to not work reliably. This is confirmed by all users and denied by @sinovoip again and again 
Fixing the problem requires desoldering components so @sinovoip is in the lucky position that if you follow his advices you void your warranty: http://forum.banana-pi.org/t/how-to-use-bpi-m3-wifi-extebd-antenna-slot/1283