Probably the grease that was applied to the mechanism to lubricate it has hardened with time. You would need to lift the deck out of the plinth and support it on something while you clean the hardened grease off with solvents, and re-apply either light grease (available from Maplins) or perhaps a thin coating of oil.
The two silver-coloured bolts visible on the top plate lock the deck to the plinth for transit when fully screwed upwards (away from the plate). Underneath the plinth, you should find two holes allowing access to the other end of these bolts. There is a horizontal metal stop on the end of each one to stop the deck being lifted out (it contacts the plinth when the bolt is tightened, locking the deck in place); these are hinged on the bolt, so flip them to vertical (in line with the bolt) and you can then lift the deck out.
This will also give you the opportunity to move the arm while watching the mechanism to see where it is 'catching'.
The deck must obviously be unplugged from the mains while doing all this!