PSNR and SSIM decrease after increasing training image

First i trained model with 800 images. During the validation i took the image from same dataset and i got the PSNR of 19. I take the test image from cross dataset. When i test it on trained model it is increased to 20.

Then i increase the training image to 1200 instead of 800 and keep other parameter same. In validation PSNR increase to 22 but during testing image of cross data set, it decrease to 19.

Why testing image PSNR decreasing even validation PSNR increase? What will be the solution to increase the PSNR of testing image?