What is your tailsitter configuration? Is this non-vectored dual motor tailsitter? I guess your motors are connected to ch4 and ch5, nothing to ch3…
If this is so, I think your elevon is too small, prop diameter is also too small. Non vectored dual motor tailsitter needs 30-50% chord size of elevon, and large diameter prop to cover elevon with stream. Otherwise better to look into vectored tailsitter.
For non-vectored dual motor tailsitter, my thread records almost a year of learning and may help.