There are a few different pre emergent options and they have different time periods that they cover based on rate of application. Some only cover as little as 30 days-ish. Some can cover 90-120. Depending on what you applied and when, you could have lost all coverage to stop crabgrass from coming up (if applied early spring) Sometimes you need to do a split application and apply again mid summer.
Don't shoot the messenger about chemicals..lol. Just giving info. Also, the thicker you can get your grass also reduces weeds that will germinate. I don't use pre emergent, but do get some crabgrass pop up in spots and some other random stuff.