They are unable to do service many spare parts, as they don't have this capability.
For example, a Polish company misdesigned an ejection seat for the MiG-29. A part that was supposed to break off did not break off when a parachute was deployed and a pilot was killed after he ejected. The company was sued and the lost the case in the court. The designer was prosecuted.
Polish MiG-29s were grounded for many months numerous times. Sometimes, it lasted for 12 months or longer.
MiG-29s were the primary planes deployed during the Baltic Air Policing M in the past. The main reason was, they are not so sensitive to dirt like F16s. In case of the latter, an engine inlet is positioned very close to the ground, so sucks in rubbish/undesirable objects from the ground. In order to make F16s operational, the runway surface has to be perfectly clean and it requires a special U.S.-made hoover. The Polish Air Force considered bringing the hoover to the Baltics a kind of a logistical problem or maybe an insufficient number of them was bought in the U.S.A. to make F16s operational in too many bases in one go.
MiG-29s don't have this problem, but they have not been deployed in the Baltics past January 2015. Make an intelligent judgement about why this policy has changed.