Bottom right?

In my opinion. The symbols stand out easily on the backgrounds you are using. The synthetic images you make are too different from the images you really want to perform well on.
I would work on trying to collect more data or do more classic data augmentation.

It would be quite difficult to generate more realistic synthetic examples. One issue you have is the square color background around the object. You should try use a background remover tool so that you just have the black lines of the symbol.