Inference Score is low AP - on different dataset

[Mahmood-Hussain]

Hello I tried the model and AP is above 40 percent but when I test it on a different dataset the AP is around 0.04 what could be the possible reasons for failure?
I trained the model on 256x256 image size and tested on 256x256 and higher sizes but nothing changed AP is still very low on the testing set. Even during I have a separate validation set but AP is good enough around 40

[Banks1y]

I also have the same problem. I think it may be caused by overfitting of the model. Have you solved this problem？