John,

Food for thought. The vent height is of no relevance to the drainability of the tub. As long as the vent diameter is at least 1/2 the diameter of the tub drain and not less than 1 1/4 inch diameter the vent can run 40 feet high...if you want it to stick 50 feet above the roof deck simply increase it to 1 1/2 inch diameter (one pipe size increase).

A plugged vent could have some effect, but I would venture to guess a hairball downstream...they sure cause my tubs to drain slowly. Also, if the slope is too shallow, there is a loss of velocity which affects the rate of flow.

Back to the plugged vent theory, slow drainage would be the least of the problems. Your trap would be sucked dry which would allow sewer gas to enter the bathroom (and you were blaming the smell on the kids, right).

Some things you might want to investigate would be the distance from the trap wier to the vent, is the vent vertical off the drain or is it a horizontal dry vent. If it is correctly vented, you could always replumb the tub with 2" from the trap to the branch drain (leaving the 1 1/2" trap) or replumb with a greater slope.

I hope this helps.