12embo,
Need more information to help.

1/ What type of evaporator do you have, can it collect oil ?
Need to know where oil is collecting.
2/ Evaporator superheat
3/ Type of refrigerant & oil.
4/ Was oil separator sizes for the load, or pipe sizes?
5/ Does oil in sump foam all the time?
6/ Discharges temperature.
7/ Is oil return definitely returning.


Usually after oil separator is sized for maximum load & is coalescer replaceable filter type like Temprite.
Oil is collecting somewhere & that will decrease evaporator performance & possibly flood back.