Bottom line, you always have to keep a cartomizer wet; however, to better address your concerns, you need to provide more info. What cartomizer & battery are you using? If the battery is variable voltage, what is it set at? What resistance is the carto? What pg/vg ratio are you using?
Ocelot is correct. The polyfil wicking material inside the carto must stay moist with e-liquid or the heating coils will quickly burn the polyfil resulting in a burnt taste. Unfortunately, once this happens the carto is ruined. You can not remove the burnt flavor, so it must be discarded.
Something that you can do is do "primer puffs". Suck on your drip tip as if you were vaping it without pressing your battery's power switch. This will pull more juice to the heating coils without activating them. Do this two or three times in a row, then return to vaping normally. Repeat this as you need to.
You might also "top off" the carto occassionally by dropping several drops of juice into the cartomizer to make the polyfil appear as it did right after filling it for the first time.
How hard are you "pulling" during your vape? A cartomizer may have a tough time trying to keep up with providing the heating coil with enough juice if you are a heavy vapor, especially if the juice you are using is especially thick (like a vg heavy juice). It's recommended to do slow, steady inhales instead of hard fast pulls. This is not a cigarette!
