Printer ink

My new printer at home, I don't use it very often, yet today I noticed that the large black ink cartridge is down to one third, and the cyan and magenta small cartridges are down to about 2/3.  How come?  Today I turned on my printer for the first time in perhaps two or three weeks.  The yellow cartridge was giving me problems making my computer think that there was something wrong with it, and thus refusing to print anything at all.  All I did was take out and insert again this freaking yellow cartridge --twice--, and the problem was solved.  I also saw that the computer thinks that the yellow cartridge is completely full.  How can that be?

Yes, the print quality is great, but this surprise with the cartridges is a bit disappointing.