If the motor turns then it's drawing some current (amps). It might not be enough to crank the motor at high enough speed to actually start burning fuel though. If the problem is indeed in the starter motor, low or no amps/current is most likely the result of a brush being so worn that it doesn't touch the terminals on the commutator, or of a solenoid terminal having deteriorated to where it has little or no electrical conductivity. It's kind of like a light bulb with a burned out filament: once the filament opens, no more current flows.
In case you're curious to know how a starter can fail the following might be interesting. Otherwise just skip over it!
I know yours isn't a Cummins engine, but it's what I'm familiar with, so here's a link to a place offering rebuild parts for a Denso/Cummins starter. fostertruck.com
All starters are more-or-less similar inside so the exploded parts diagram and the color pictures of the individual parts that wear might help you visualize how a starter could fail. For that matter, if you're brave, it might give you the inclination to disassemble yours just enough to peek inside. Though you might not want to tackle rebuilding it yourself, a peek inside might reveal something that is obviously worn. I sometimes do this to gain confidence that an item is broken and that spending a chunk of money to replace it is the right thing to do. If I can't find anything wrong then I may be able to re-assemble it. If not, I was at the point of guess-and-check replacement anyway so there's really no loss for having definitely broken it by peering inside before buying the replacement.
First look at the photos of the brushes and the brush plate. On the brush plate there's a coiled spring pushing each of the four brushes in toward the center. The brushes on the left and right bring ground/battery negative to the motor; the brushes at top and bottom bring battery positive.
Now look at the photo of the armature. Those shiny copper fingers at the left end are the "commutator." The brushes in the brush plate are temporarily held back so that the commutator can be inserted through the center of the brush plate. The springs then hold the brushes firmly against the copper fingers, carrying electric current into the wires wound around the armature. The armature/commutator literally grinds away the brushes as it spins. Eventually so much of the length of the brush has been ground into dust that the brush can't be pushed in to the commutator any more and it doesn't make good contact. The motor's speed/power drop off quickly as the last bit of a brush is consumed.
When one brush gives out the motor can work only half as hard and engine cranking will sound weak. Soon the brush on the opposite side will wear out too and then the motor won't run at all. Maybe this could be what has happened with yours. If you can lift the back cover off the motor enough to peek in at the brushes you might be able to see whether this is the case. Be careful not to lift so high that the brushes pop off the commutator: it can be tricky figuring out how to hold all four of them back at the same time so the thing can be re-assembled!