@Smilee, there are many possible causes. Air flow depends in part on the atomizer. Some take in air from the bottom and the little pinholes on the connector, others only one of these. There are truly many atomizer designs. The XL can supply air from the bottom to the atty's center tube, or from above to the pinholes if the inner o-ring is removed (or destroyed from wear;-) ). You might have to take that o-ring out for some atomizers. See my drawing. The XL's motto is "Every atomizer works. Some are an adventure!"
Even then, the space between the carto's outer walls and the chimney's inner wall is important, and some atties are very "fat" so little air gets through. That's the case with a 306 or an i06 with a drip-tip. As juice leaks past the o-ring, which happens when you have to squirsh hard, that space gets full of juice, and air-flow is reduced more.
Are all your atties the same type?
Or you atomizer could have whichever holes it sucks air through getting plugged. Maybe time for an atty bath in alcohol (spirytus)? Cartos like the Boge LR start out with fantastic air flow, and after a few days hard XL vaping start to get clogged with poor airflow, and there's no cure I know of, other than putting a fresh one on.
An XL bath might help too, and take the o-ring out before the bath anyway, so you don't have to take your kitchen plumbing apart to get it back ;-)