TL;DR?: update your antivirus, check if anything is blocking us, and check that your files aren't open in another place. This'll give you the best chance to ensure things work as swimmingly as possible. 

Oh no! So you've selected your files, chosen the type of transfer you'd like, maybe entered a recipient or two, clicked 'Transfer', and then... nothing? If you're having trouble starting a transfer, the most common reasons for this are antivirus and blocking software.

Antivirus and blocking software can be like an overprotective parent; intentions are great but sometimes they just get it all wrong. If that happens, your software will cause problems with both uploads and downloads on WeTransfer, because the software doesn't trust us. Our service will be actively blocked from connecting with your computer.

Antivirus

Despite these challenges, please do not go deleting your antivirus just yet! The great thing about antivirus software is that they update quite often.
Updates will add newly found viruses and tweak the software, which may have a positive effect on how well our service performs in combination with the antivirus software. That's why we ask you to check if your antivirus software is up to date (not all antivirus software automatically updates, and we can't see what kind you are using) before moving on to other suggestions such as adding our domains to your trusted websites.

Some software uses a web shield (Avast, for example) which we know can block our service, while others use HTTP checking or Bitdefender, the last of which works to completely disable our service.

Blocking Software

Something completely different but with the same result (our service not working properly) are browser extensions or plugins. Script blockers for example, block parts of our interface, which renders it useless. Ad blockers can detect advertisements, but sometimes also disable parts of our interface, if they interpret parts of the code incorrectly.

Open Files

Another thing that could be preventing your transfer from starting is that you've got your files open in another program or application as you're uploading them. If you've got, for example, a video file open in some editing software as you're uploading it, the file won't be uploaded and the transfer will fail. Double-checking that you've closed all programs or applications related to your files is a good move.

The Bottom Line

We test all of our service with all major browsers, but not in combination with other apps. The reason for this is that we simply can't test for all possible combinations of software on individual computers. We know that certain antivirus apps interfere with our platform, but because they change all the time (updates, new features) we can't pinpoint the exact cause in many cases.

We can't write our service to work around antivirus software, because that's exactly what antivirus software is for: to defend your computer from unwanted software accessing it. Sometimes it just works too well! Just think back to the overprotective parent comparison we made at the start of this article.