currinson,
Welcome to TBN. Hope we can help.
Did you have any weight (like your
chipper) on the hitch when you were trying to lower it?
And did you open that "lock" under the seat fully? It's actually a valve to control how fast an implement drops, and only serves as a lock when it's completely closed. If you only open it a little, the fluid can't escape very fast. If you didn't have a lot of weight on the hitch, it might be dropping very slowly giving the appearance of not dropping at all.
Here's a longer explanation:
The rockshaft is the horizontal splined shaft which has the two short arms which pull up on the lower links of your hitch. That shaft is supported by two bushings.
When you pull up on the lever to raise your hitch, a piston is forced out of the rockshaft valve against a pawl splined to the center of the rockshaft.
When you push down on the lever to lower the hitch, a valve opens to allow the hydraulic fluid escape from the cylinder through the "slow drop" valve (the one under the seat).
If your hitch won't drop, then either the fluid can't escape through that slow release valve, the piston is binding the cylinder, or the rockshaft itself can't turn in those bushings.