Need help with the logistics of wearing a vest! So MFA, I have been having some trouble when wearing only a vest with a dress shirt (tucked in). The dress shirts fit quite well, not tailored but length and bagginess is not an issue when un-tucked. The vest fits very well, one has been tailored the other fits just as nicely... The problem is; after a few minutes (or as soon as I sit down, usually) some of the tucked shirt becomes 'slack' and appears above my pants and below the vest, sort of pinched there. 

Is there something I'm missing? A way of tucking it in that somehow prevents this? Or is it normal and I am being too picky?

*Didn't provide pictures because I figure it is a simple enough concept*