As the battery runs down, and the carto empties, you will get less vapor. The remedy for this is to charge, or switch the carto (I'll give you a little hint, you can actually refill those cartos if you just buy juice from somewhere. They won't tell you that, buy I've been using the same cartos for weeks now, they'll work for a while).
If neither of these things are the case, you might have a bad connection. Be careful how you screw in the carto. They can sometimes be finicky. The crackling is perfectly normal. As for the difference between the disposable and the rechargeables, it's a completely different set up.
But to be honest, I don't much care for he Blu brand. Volcano is my brand of choice, not that there aren't many brands out there that are good. Volcano offers the same basic set up, but the customer service is out of this world, and shipping times are very fast. Plus they offer larger batteries. In my experience, the products are very high quality.