It strictly depends to the mechanism of main and co reactions. For example when A=>B=>C, and B is main product, increasing the length of the reactor can promote B=>C reaction, lead to decreasing the selectivity.

thank you sepher sadighi

Another aspect is fouling of catalyst which drastically reduces the catalyst activity. For example, in FCC riser reactor, heavy crude oil is cracked in presence of ZSM5 catalyst to produce gasoline and diesel at ~ 1000K. If residence time is higher (long riser)  in this high temperature environment, coke may also form which blocks the active sites of catalyst surface. A re-generator is always connected to riser reactor to revitalize the catalyst by burning off the deposited coke material.