Given the shape of the balloon (I'm assuming it's pretty spherical or cubic) and the leverage those motors will have around the center of mass, I'll bet most of the energy is going into rotation (culminating in a balanced fight with gravity). To deal with this problem, dirigibles/zeppelins had long cigar/torpedo shapes. My bet is that if you just created another identical motor board you could mount one on the "nose" (wherever you choose that to be) and opposite on its "tail." Then it's balanced and it won't matter that it's spherical. Get them push/pulling the same direction and voila (off to the races)! 

You'll probably figure it out with more experiments. Does the whole thing have neutral buoyancy? It would be fun to see a video. 

Yeah there's neutral buoyancy. I'll take a video once it actually lifts!

Are your propellers going the correct (most efficient) way?  I'm not being a wise-guy - the lower motor looks like it's supposed to be doing the lifting, and it's facing downward.  Most toy props I've seen are designed to be pullers, not pushers, and probably aren't very efficient in reverse.

Good point, the ONLY reason I designed it to face downwards is just because the toy blimp I bought  has the "lift" motor facing downwards as well. I don't know enough about air flow design yet to comment. 

I def don't feel enough lift coming from the motor I'm using- either it's not powerful enough, or more likely, it's not fast enough. I just ordered a sampling of motors w different rpms and will be experimenting in the coming weeks on both rpm and motor placement! 

You removed and flipped the lower propeller?

@Richard Hogben yes