One of my sites https://wordtohtml.net gets great traffic, the issue is only 0.1% of visitors convert to a paid plan.
I think the main issue is the free version does most people so they have no need to go PRO. The PRO version has lots of cool features but it just does not seem to be encouraging enough people to sign up.
I know myself when a company has a free version you always try to avoid paying no matter how good it is, eg with dropbox i use it all the time but when i run out of space I just delete some files instead of paying for a plan.
Any suggestions? Should I kill the free version and just make everyone pay? Remove more features from the free version? Suck it up and just concentrate on the users who do pay?