Mask wont export even when i merge layers

So i tried to export a file ive been working on for my clothing brand, but now the problem is that when i try to export it, a layer mask that im using won´t export with it. Even when I merge the layers the mask somehow still misses when exporting the file. I tried to change the document settings stated in some other discussions, but that all didn´t work for me. Does anyone know what the problem here could be?

In the files I showed the document in photoshop, the exported product (which already shows it didn't show the layer mask) and the file on a tee shirt in this example. 

Thanks in advance.