The 1900 has a 2-stage clutch. The same pedal engages both the transmission and the PTO. At times, particularly when seldom used, the PTO clutch will "stick", could be rust, and it won't matter how far or hard you press the clutch, it will not turn lose.
At a shop they can use a machine that will apply pressure to the PTO while revving the engine that will, sometimes, break the PTO clutch lose and will work fine after that.
It is possible to do this yourself, but it is not always easy or safe. It may work by "popping" the clutch while the motor is revved the PTO is engaged with the mower attached ... but, if you have a 4' mower, it will pop the shear pin before it releases a stuck clutch.
If you have a 5' mower (hope you have weight on the front of that 1900 if you are mowing with it) you may be able to put a hardened bolt in for a shear pin and "pop" the clutch. If you try the hardened bolt in the 4' mower, you may damage the mower since the 4' is no where near as heavy duty as a 5' or 6'.
If you have a 5' or 6' for another tractor you could try it with the heavier mower, just don't try it with the mower lifted "off the ground" as the 1900 is not designed for that much weight (yes, we used to have a 1900)