The electromagnetic field strength is proportional to the number of coil windings. The copper wire in a coil has a thin film of insulation on it. If the coil has been stressed and overheated, it can melt the insulation and numerous windings become bonded together, effectively reducing the number, and thereby the magnetic strength. Also, a reduction of voltage to that coil will have the same affect, which can occur through oxidized connectors. You can test this by running a jumper wire directly from power source to the coil and see if the strength improves.
Also as the previous poster mentioned, the coil sleeve could be worn or melted causing restriction and an apparent loss of strength.