Carb eq'ing betweentaps

I I was wondering about carbonation and how it relates to having more than one beer on the the same system. When I put my homebrew I. A keg it always seems like my carb levels are low. So I will crank up my psi on on keg and turn the others off to help boost the level for a few days and then will cut off the pressure to that keg and will turn down the psi to serving level and turn on the other taps. But when I bring the new keg back on tap the carb level never seems to go up. If I have 3 beers on tap, do they eq the carb levels between them? Even if I carb with priming sugar the carb level never seems great. I have checked for leaks and could not find any. So any ideas out there

Pressure could certainly equalize between multiple kegs if there isn't a check valve or the like to prevent back flow.

I assume you are using a Wye / Tee / Manifold to split one gas source off into multiple kegs? Using individual shutoff valves for each gas line (sometimes these will have built in check valves), or just pulling the QD off?