It takes a little time to figure out how often to replace the cartidges to keep the carb level just right and flowing well. It depends upon how fast you drink, temperature, and other factors. I started using just one cartridge, then putting a second in when it slowed down. Usually two-three cartridges per bottle, depending upon how fast it was drunk. This was for already carbed beer, and I used the whole amount of sugar for priming, not the amount they recommend in the instructions.
It gets better with time/practice. Not quite as easy as a full kegging system, but easier than bottling and cheaper than buying a kegerator!
Edit - re-reading your initial post, I thought of one more thing. I think some carbonation comes out of solution when the beer level is low, like at 1/3-1/4 or so. If we weren't drinking it quickly, and it was getting close to needing a new cartridge, it'd be a little flat after several days. Makes sense, as it's not at 15 psi anymore, maybe 4 or 5. But putting in a new cartridge would always make it great in a few days. Some CO2 gets absorbed, some remains for pushing the rest out.