The check engine light soon light keeps coming on on my 97 plymouth voyageur.last time it did it, it was a stuck pollution valve, ( it had an evap code ) so they unstuck it, it is now back on why does it do this?

The first thing for you to do is take vehicle to automotive parts store where they offer to read the OBD computer fault codes for FREE (they may also suggest what item(s) caused the fault code). If the vehicle is otherwise running normally, it may be minor problem, or in some instances just a false alarm.
If Check Engine light is on and it was not on for several days of driving after they fixed the stuck evap (emissions) valve, then the On Board Diagnostic (OBD) computer has detected possibly another item not functioning properly. It can be one or several items giving a fault code (the OBD monitors many items); It may even be the same stuck valve; it may need replacing if so.

the check engine light soon light keeps coming on on my 97 plymouth voyageur.last time it did it, it was a stuck pollution valve, ( it had an evap code ) so they unstuck it, it is now back on why does it do this?
If the code was for the "pollution valve(?)", but you unstuck it, the problem is likely not the valve, but rather the passages that lead to that valve are clogged (if I'm correct and you are refferng to the EGR valve?). Maybe you should replace that valve.
And cleaning the passages is a major bit of fun. And shouldn't be attempted by a DIYer. You can create more problems than you fixed. Especially on an older vehilce such as yours.

But, before you freak out, the problem could also be the "pollution valve" control.
Which valve are you calling the "pollution" valve?

Have a scan done; post the codes here. We can't point you in the right direction if we don't know where to start.