The cams have to slide the clutch yokes up and down to engage. They have spring loaded plungers to push the yoke so that if the dog clutch is on a high spot you can still shift it. Also the cams have a roll pin in the shift shaft. Check the plungers for binding up or broken spring. Also yoke wear. Plunger spring tension is critical for shifting. I believe it will be the lower yoke trying to go in the up position that will be the problem. And, If you have no experience on planetary mixers, Find someone who does. They are not as simple to work one as one thinks. There have been many torn apart to never be reassembled again.
eritech now states that it is 1 and 3 not engaging. From top to bottom the gear ratios are 3.11, 1.06, 1.64, and 2.88. So first gear would be upper clutch up and lower clutch down. Second gear would be upper clutch up and lower clutch up. Third gear is upper clutch down and lower clutch down. Forth would then be upper clutch down and lower clutch up. So the common fault comes with the lower clutch not fully engaging the dogs in the down position. This can be just a foreign object in the dog clutch to a score or dirt on the yoke shaft to plunger spring. Every one requires a form of tear down.
This mean you have to mark certain parts for indexing on reassembly, Have the right spanner wrench’s, drifts, chisels, etc. Again, I encourage you to have a Hobart tech. look at it..