The shifts are also controlled by the governor and if this sticks then it will not upshift from 2nd to 3rd.
You can access the governor and remove it for inspection and/or cleaning through the small hatch on the R/H side of the extension housing
There are 2 slotted head screws holding the cover. Once removed there are 2 bolts (can't remember what size they are but close to 7/16 socket probably) holding the governor body on.
When you get it out the valve should move back and forward freely
Be careful not to drop the bolts in the extension housing when removing or refitting the governor.
Gary - 1958 Star Customline and 1940 Ford Deluxe Hotrod