Are these old cartos or new blanks? You probably aren't doing anything wrong the cartos are just burnt. Instead of blowing out the cartos, turn them upside down, apply a little pressure to the side of your bottle, stick the dropper into air tube...then release the grip on your bottle. The vacuum generated will suck that
juice right back into the bottle.
If you have this going on with a couple of carts, reverse spin the worst one, then take a pair of tweezers and yank the polyfill out. If you don't see an obvious burn hole unravel the polyfill, check the wicking material and the wick tube...trust me...you'll find one.
As long as the wicking tube isn't compromised....there really is no way for
juice to flood the air tube from the polyfill. It can only happen when a hole has developed.
As for carto lifespans, they can only take so much and after awhile the a section of the atomizer will simply stop functioning. This is why you'll hear most juicers say they use (non-burnt) cartos until the vapor production starts to drop off.