It's not unusual to get an occassional "bad" carto. Quality control of these tiny heating elements leave something to be desired. However, since you've had a few in a row with disappointing results, I think it be more likely something else is at work here.
You mentioned that you were having some problems visualizing if the carto is properly filled. When I fill my cartos, I put a wood tooth pick into the air hole. This blocks any juice getting in there and eliminates the need to blow the air hole out later.
I drop juice onto the polyfill material while spinning the carto around in order to get a wider dispersion of juice. When the filler looks like a "wet slushy drink", I then take the carto and shake it like a thermometer. This simulates the action of a centrifuge by forcing the liquid down into the bottom of the filler material. After a few shake downs, I add more juice until it again looks like the slushy drink. Shake it down again. I repeat these steps until the filler will take no more, and remains looking wet. You can then remove the tooth pick and place your drip tip on the carto. It's ready to vape.
Another possibility to consider is you don't care for
low resistance cartomizers. I know that I don't. I find them to taste harsh and to have a burnt taste with some flavors. I've found that I prefer a "cooler vape". I use standard resistance cartos (2.0 - 2.4 ohms) on fixed voltage batteries, and 3.0 ohm on variable voltage batteries. I've also found that the longer drip tips of 1 1/2 inches long allow the vapor to cool a little more due to the distance that it must travel.
http://www.myfreedomsmokes.com/parts-accessories/510-parts/510-miscellaneous-accessories/510-long-skinny-drip-tip-aluminum/
http://pinkspotvapors.com/shop/hardware_accessories/freudian-tip/
By the way, if you are using your cartos solo (not in a cartotank), don't be afraid to frequently top off the carto with additional juice as you vape. Once a carto gets dry and burns, you'll never get that burnt taste to leave. You'll have to discard of it.
Consider trying a cartotank. Each time you vape the tank feeds more juice into the carto automatically, so you will no longer wonder when to add more juice.
http://www.e-cigarette-forum.com/forum/blogs/baditude/3710-5-cartomizer-tank-setup.html
This link will also explain about cartos and cartotanks, along with some other juice delivery devices:
http://www.e-cigarette-forum.com/forum/blogs/baditude/3230-1-proper-terminology-carto-tank-what-primer-juice-delivery-attachments.html