SM signal get worse

HI, on some ubnt devices signal become worse (from 60 to ~70)
wiev to bs are clear . This problem are on almost  all Ap . First AP software was 3.3 now 3.5 and 3.51
What can be 


What models ubnt m5 , small range beams16 and 19 works realy bad for me, I am replaceing them with force180 and geting 5 to 10 more dbm depense on the freq. beams 300 and 400 works ok!


m5 xm ,  beams16 signal are worse on 2-3 dbi  comapring to nanostation m5, beams16  we have problems witch lan drop after 3-7 days on usage but in 3.5 this problem was fix