Not so likely to be the seal - more likely to be the window coming loose - although it could be a combination of seal going slightly out of shape and the door shifting slightly - but you can adjust for that and don't need to buy a new seal.
Grab your window glass and waggle it then do the same with the passenger side to compare. If the driver's side glass seems loose it's just a case of tightening the glass bolts. This often happens if people push the glass to close the door.
If the glass is rigid then check the door closing body lines to see if the door has dropped or is out of line or the striker bar has worn etc., allowing the door to close slightly ajar. The striker bar can be adjusted to pull the door back flush with the body.
If all the above is Ok then you can adjust your glass position by adjusting the door sub frame position in all directions including the all important tilt inwards which presses the glass into the seal. See my sig strip "window cable" or the Mk1 Knowledge base for details - there are full adjustment details in there.