Ask HN: How to onboard users better?

I have a SaaS that lets people extract data from video files. It's simple enough to use for 1/5 users but the remaining 4/5 seems to never get it right (they are doing the wrong steps) Their mistakes are forming a pattern.

My next immediate step is to improve the UI so that they don't make the same mistakes.

However, overall, I find that some video files won't work properly without me guiding them. I have to literally tweak the product so that it will work out the way they want it.

Now upon sign up, I offer them a way to get in touch with me if they run into trouble. 4/5 users don't seem to reply or even reach out, they simply use it, try it and leave.

I want to improve this by helping them have a successful experience with the product. How can I improve the onboarding process so that I won't have people leaving because of unsuccessful jobs (either because they didn't do it the proper way or the product is simply running into bugs which I need to fix).

Also, should I be doing this for every prospect without qualifying them first? I spend all day and night helping new users as they come and request help, but sometimes I find I do all the work and fixes to find that they are not interested or it's too expensive and that their department doesn't have the capital.