Likely as not you just need to give the window a proper cleaning.
That includes simply lifting the fuzzy track at the bottom of the glass and cleaning under it.
Pour water on the glass and observe the drains, they should run freely with clear water.
Also, check the track from the inside during a hard rain, if the track is wet or filling with water, then the drains are plugged, at least partially.
Worst case scenario...
A whole new window is under $200 and about a 3 of 10 on DIY scale.