COMBINE/MERGE FILES ON EDIT PAGE NOT WORKING

Today I went on to use Adobe Acrobat Online service for merging two PDFs from my PC. I used the 'Combine Files' tool/option to do so. Right on the page (aforementioned), I clicked on 'Select Files' > 'From My Computer', and on first attempt, it did work (though there was a little glitch of which I'll write afterwards). So I was able to merge two PDFs resulting in a combined one. I downloaded it. Then, I tried to do the same sort of thing with the same process I've already described earlier, but it didn't work. After that, on multiple occasions, I've failed miserably, I don't know how. I'm attaching a graphic image that may help you with your solution to this problem.

Thank you