Watermark

I am using Lightroom version 5.2 on a Windows 10 system and I have been exporting my photos with my signature (.PNG) without any isses but if I try tom export a photo with another .PNG instead of my signature, it will not do this. I shows the watermark in the preview but when I actually save it to my PC, the watermark is not there. I dont do anything different when exporting with my signature so I cant understand why using a different .PNG is an issue. Can someone help me figure this out as these are photos I need to showcase on my website for sales. Thanks