is the very back of the pto shaft splined area chewed or damaged in any way.. .. even a SLIGHT twist will cause a hard engagement.
also.. is there any slop fore / aft of the pto shaft in the bearing carrier? if so, the wear ring may have slipped or the bearing is loose. this will let the pto shaft move fore and aft and in many cases 'walk' itself out of engagement.. this usually chews the end up. Also.. and especially if the driveline is too long.. or not lubed, the pto shaft end ( aft end at the coupler int he tractor ), can beat backwards ad MUSHROOM deform, and be difficult to engage. use a mic on the splines front to back. in some cases.. taking a flat file to very mildly mushroomed or slightly twisted splines will make them wor.
if the pto shaft looks true, next look at the sliding dog clutch.. part #12 in the diagram I'm attaching.
here's the parts listing to go with the diagram.
ps.. you can remove that or peer in to inspect it pretty easilly.
good luck.. hope it helps
soundguy
1 1 NAA770A Support, PTO, NAA, NAB, No Longer Serviced, Years: 01/01/1953-12/31/1954
1 1 NCA770A SUPPORT Support, PTO W/4-Speed Transmission, No Longer Serviced, Start Year: 01/01/1955
1 1 NDA770B Support, PTO W/5-Speed Transmission, All Purpose & 900 Row Crop, No Longer Serviced, Years: 01/01/1955-12/31/1959
2 AR NAA722A STOP Stop, PTO Shifter
3 1 9N719 Rail, PTO Shifter, No Longer Serviced, Years: 01/01/1953-12/31/1954
3 1 NCA719A SIDE RAIL Rail, PTO Shifter, Except NAA, Start Year: 01/01/1955
4 AR PL PROCURE LOCALLY Washer, Lock, 3/8"
5 1 PL PROCURE LOCALLY NUT,3/8-24, Hex, Start Year: 01/01/1953
6 1 797234 SPRING SPRING, PTO Shifter, Rail Mesh, NAA, Years: 01/01/1953-12/31/1954
6 1 83407013 SPRING SPRING, PTO Shifter, Rail Mesh, Except NAA, Start Year: 01/01/1955
7 1 145222 BALL BALL, 3/8", Steel, Start Year: 01/01/1953
8 1 PL PROCURE LOCALLY BOLT, 3/8-16 x 1 1/4", Start Year: 01/01/1953
9 1 233274 SEAL Seal Assy., Transmission Countershaft, Start Year: 01/01/1953
10 1 NLS No Longer Serviced Fork, PTO Shifter {NAA720A}, Years: 01/01/1953-12/31/1954
10 1 NCA720A FORK Fork, PTO Shifter, No Longer Serviced, Start Year: 01/01/1955
11 1 128999 O-RING O-Ring, -111, 70 Duro, .424" ID x .103" Thk, Shifter Fork, W/NAA720A, Years: 01/01/1953-12/31/1954
11 1 14457281 O-RING O-Ring, -114, Cl 7, .612" ID x .103" Thk, Shifter Fork, W/NCA720A, Start Year: 01/01/1955
12 1 NAA717A SLEEVE Sleeve, PTO Clutch, 2.16" Overall Length, No Longer Serviced, Years: 01/01/1953-12/31/1954
12 1 NCA717A SLEEVE Sleeve, PTO Clutch, Except NAA, 2.16" Overall Length, Start Year: 01/01/1955
13 2 9N754 RING Ring, PTO Bearing Snap, Years: 01/01/1953-12/31/1954
14 1 210064 BALL BEARING BEARING, BALL, Assy, PTO Shifter & Shaft, Optional W/B & C, NAA, NAB, Replaces 9N715B, Years: 01/01/1953-12/31/1954
15 1 9N723 LEVER Lever, PTO Shifter, Except Offset, No Longer Serviced, Start Year: 01/01/1953
15 1 312844 LEVER Lever, PTO Shifter, Offset Only, No Longer Serviced, Start Year: 01/01/1959
16 1 60518S RIVET Rivet, Button Hd, 1/4" x 1-1/4", Start Year: 01/01/1953
17 1 NAA721D Plate, PTO Shifter, NAA, Replaces P/N NAA721C, No Longer Serviced, Years: 01/01/1953-12/31/1954
17 1 NCA721B Plate, PTO Shifter, Except NAA, No Longer Serviced, Start Year: 01/01/1955
18 AR PL PROCURE LOCALLY Nut, Hex, 7/16"-20
19 2 PL PROCURE LOCALLY Stud, 7/16"-14-20 x 1.68, NAA, Start Year: 01/01/1953
2 PL PROCURE LOCALLY Stud, 7/16"-14-20 x 1.68, Except NAA, Start Year: 01/01/1955
20 2 PL PROCURE LOCALLY Stud, 7/16"-14-20 x 1.3/8", NAA, Years: 01/01/1953-12/31/1954
4 PL PROCURE LOCALLY Stud, 7/16"-14-20 x 1.25, Except NAA, Start Year: 01/01/1955
1 PL PROCURE LOCALLY Pin, 5/8" x 1/2", L/PTO All Purpose Heavy Duty Industrial, Start Year: 01/01/1955
1 353081S BALL BALL, 5/16", Steel Offset Only, Start Year: 01/01/1959
1 NDA784A RETAINER Retainer, PTO Countershaft W/5-Speed Transmission, All Purpose & 900 Row Crop, No Longer Serviced, Years: 01/01/1955-12/31/1959
1 C0NN4022A Vent Assy., Air, PTO Shifter Plate, L/PTO, All Purpose & Heavy Duty, No Longer Serviced, Start Year: 01/01/1955
1 9N4131 GASKET Gasket, Shifter Plate Cover to Housing, Start Year: 01/01/1953
1 86613266 CUP Cup, Transmission Countershaft Rear, Bearing W/5-Speed Transmission, Years: 01/01/1955-12/31/1959