Daikin rooftop defrost problems

I have five Daikin rooftop model UATYP 560AY1 with same problem :
in winter , in HEATING mode , defrost program is too short and ice cover the condensers .
( all the rooftops are mounted in same conditions )
If i change sensor positions on condensor , the defrosting period is the same .
I want to increase the defrost time ( if is possible...) , but i don't have technical manual for PCB .
Please help me with some informations ....

Thanks in advance