It's possible if the bottom of the gearbox is full of mud, but I think, quite exceptional.
You can remove oil and open the biggest yellow cap under the gearbox to check how dirty it is, and to clean it.
Generally it's not very dirty, but if it will be, you can fill with new oil, and change it again after some hours of driving.
Then, it should be OK !
Good luck
AL1
Salmson, la sécurité dans la Vitesse !