COUld be any number of things.

Fresh air or lack of it will certainly cause problems, but you do NOT need cO2 to fix it. In fact, if your problem IS circualtion, flooding the space with cO2 will only help to make the problem worse.

Co2 will be good if other conditions are good. But it wont help a bad grow get better.

Here are 28 days old plants from seed I grew a couple years ago...under good CFL and good temps. One is indica the other is sativa... the one with the tape measure is at 31 days.

You might reconsider adding anything until you figure the issue out.

Good Luck.