I have had this issue before with a partner. Issues like this will always come up when one person is more into the project then the next. I have made this mistake before and the way I was able to fix it was when I told him that based upon the extra work that was put in we need to come to a new agreement on percentages. He at first was ticked off at me even bringing it up as we 50/50'ed the initial invetstment but after I was able to make him realize that it was not the money that was going to make us successful it was the work involved and how much I was really doing we came to an agreement that for 6 months I was at 65% versus my initial 50%.

I think you really need to nip this in the butt a.s.a.p. because either they do not have as much experience and simply cannot do as much work as you or they simply do not want to do the work. Whichever it is you need to see if this is the person you really want to partner with.

Good Luck with your j.v.