ABS+PLA flashforge

[AmyInNH]

You can't print PLA on top of ABS and vice versa, if that's what you're trying. From your description "the first layer of PLA is ok, then when it switch from ABS to PLA again, ..."
If instead you mean you always have trouble when changing from ABS filament to PLA filament, likely you have a small amount of ABS still in the extruder and the PLA temperature is not hot enough to push it through. Preheat extruder to ABS temp and push through PLA to push through any leftover ABS, then back down temp to appropriate PLA temp. If extruder is blocked by ABS, pushing melted PLA into the blockage will cause it to back up into the PET.

Also, when you say filament is blocked between PET and extruder, I can tell you on my FlashForge, the two extruders are not perfectly level. So, if you're running both extruders and one is putting down a higher level of plastic than the other, quite possibly the plastic in the lowest extruder is right up against the higher layer of plastic from the higher extruder, and with no where to go, the plastic is backing out into the PET. I haven't dual extruded yet, but I can tell you I have to relevel the bed when switching from left to right extruder use and vice versa.