Ask HN: Startup Co-founder working on a lot of other things. Advice?

Some background: I am currently working on a startup, and there are 3 of us. We're in the UK. We've been at this for about 3 months now.

I'm technical (coding etc), the other two are more business focused. They are planning the product out, I'm building it for them (it's a dashboard basically, showing charts and graphs, fairly standard stuff from my perspective).

Division of shares is 20% (me), 40% for each of the other founders (Mr X, and Mr Y).

The startup is Mr X's idea. He convinced myself and Mr Y to leave our very nice jobs and work on this full time. Mr X told us he would be doing this as well, but he has not. He is heavily involved with a number of other startups (I think 4 others).

Myself and Mr Y are banging our heads against the wall. We get Mr X on a call for an hour each day, which he is often late for as his other calls run over. Sometimes he is late to other calls as our call runs over.

Things that need to be done by Mr X just aren't getting done, it's me and Mr Y doing most of the work at the moment.

From my perspective, I think the idea is good, the industry is interesting to learn about, and I'm happy to sit and build out a dashboard, but it won't work without Mr X's full input. He's a very smart guy, and very knowledgable about this industry, but he's just not pulling his weight.

We are pre funding. Myself and Mr Y are currently relying on our savings, Mr X I think gets income from his other projects. I think we will be able to raise money, Mr X has a good track record in regard to funding.

We have tried:

* Asking Mr X to give up equity until he is on board full time. He would look into it...

* Straight up saying on calls, "we need you to stop working on your other projects and focus on this. we quit our jobs for this". Lot's of "yes that's the plan" etc

* Threatening to walk away if things don't change. Mr X gets very upset, promises to fix things...

So, what the hell do we do? All advice is welcome! Thanks