Seleukids were waging wars against Ptolemies very often.

The biggest problem for Seleukids was the east - those provinces were so far, that full controll was almost impossible, and during every period of weakness some portions of "far east" were lost.

In the begining of II century bc it seemed they are winning - in Panion battle they crushed Ptolemies and were in the best position to became most powerful hellenic state.

But loss of Magnesia battle to the romans and loss of all asia minor that followed was deadly wound. From this time Romans were keeping status quo between Sel and Ptol while Sel were being destroyed by coups in court and Partians.