I wasn't going to get in to the "100% VG ejuice", but since you say it's thin... I will. So, obviously, if it has nicotine in it... and/or any flavoring... it can't be 100% anything. I'm guessing it's just max VG as I had been calling it... but if it's really thin... then it either has a lot of other stuff in it to thin the VG down. When you think of 'VG', think viscous goop. I'm not talking molasses, but I'm thinking thicker than 10W30 auto oil.
Those high resistance heads sure aren't helping, as they are causing your wattage to start at somewhere around 7 Watts with a freshly charged battery and then it's all down hill from there. For something like this, I would be pulsing the button on for 2 seconds, off for 1 and then back on for 2 seconds and then start my puff... just to make sure the coil is good and hot first. The pulsing is just not to put undue stress on the battery and/or to prevent hitting any type of puff time limit.
IMHO, buying anything else down this road would likely just be an overpriced waste of money. I will say though, that the 900mAh battery may actually be able to handle a higher output than the 400... but I doubt it.
I would (and did when I first started) be looking at better (not more expensive) equipment. For what you have already spent, you could have a decent clone mechanical, rebuildable tank, wire, Rayon and batteries to last you a year. . . you may have been able to spend that same money on a variable wattage mod (MVP2 kit) as well depending on where you shop.
Don't sweat it though, just learn from it.
There is tons and tons of great info here online, and quite literally thousands of people to help guide you... I suggest reading and then asking more questions as they come to you... or finding a local shop that will let you try a few things out or that has people who will let you try a few things out before making that next purchase.