Like slowrev suggested, look for about 14 volts at the battery when the engine is running. To answer your specific question though, yes, voltage can be present where amperage is not. It takes both to charge a battery. If you do have the proper voltage at the battery when the engine is running though, I wouldn't bet on low amperage. It is possible for part of the rectifier system to go bad and amperage to drop, but in a tractor where the run time compared to cranking is so long I doubt that it would be abvious. Back to what the problem can be though. Let's say you do have the voltage and amperage going to the battery, but still a cranking problem. It's starting to look like a battery problem. Before confiriming that, make sure you have good, clean connections, and check the condition of the wiring to the battery. I've seen wires that looked good on the outside, but were corroded on the inside to the point they wouldn't conduct. I'd find those with the voltage meter. No load, voltage was good, but under a load the voltage would drop considerably. The voltage meter will not read the amperage, but under the bad wire scenario what was happening was that the wire was not allowing sufficient amperage to flow. All of this, or any of it could be happening in your cranking problem.
I recently had a charging problem on my tractor. It was as simple as a loose connection on the output line of the alternator. I hope yours is as simple. Do some checking and get some more info. I'm sure someone can get you on the right track.
Tom