Presentations not loading in meeting

Recently I have had trouble uploading my presentations in a meeting. 

I share/upload via the upload button - but participants in meeting cant see presentation. 

Caused a problem in a number of meetings recently - could present via proxy luckily but very clunky and awkward to have to ask someone else to keep changing slides for me - and unprofessional.

I have a load of presentations in the next 2 weeks so getting worried

What could be the problem - and how do I resolve it?