Okay.

Example 1: Doed -
Max Weight: 870.4
Used Weight: 437.2
Refuses to harvest while: 1: Being carried by quetz. 2: Being set to wander on the ground. 3: Being set next to a resource node (on top of)...

Steps to make it harvest again:
1: Keep it next to the resource node it's refusing to harvest
2: Remove...200 stone to make the weight 400-ish and it harvests again... until...back above 430 weight.

The same occurs for the anky and the beaver (the only other two I've tested thus far). They reach a certain weight threshold (typically about half their max weight) and refuse to harvest until weight is removed. They're nowhere near max weight, either. The doed, for example, is at about half.

Modlist: Structures +, Advanced Harvest, Dino Mindwipe, Homing Pigeon, Advanced Rafts, HG Multi Storage, DPs Tranquilizer Rifle, Upgrade Station, Classic Flyers, Snappy Saddles.