I have seen the steel supply line to the speedo adapter rub thru on the firewall small and end up with a vacuum supply leak preventing the speedo adapter to shift to high.
That speedo adapter once the line is disconnected and cable pulled off will come right out to bench check it over.
The adapter needs to be checked out that you can manually shift it...move the little flag lever to shift it from lo to high.
Also check that again by sucking on the brass line connection fitting... duplicating engine vacuum being applied.
If it does not move the lever
Also if the lever doesn't move when sucking on the fitting the small rubber diaphragm is probably stiff or torn.
Speedo cable needs to be checked out that it is not damaged and properly lubed.
Same with the speedo head. You will see that small 1" stem sticking out of the bottom of the speedo,,,it unscrews and has a felt wick in it....put some light weight oil in it to lube the speedo shaft and bushing.
You can use you fingers to make sure the speedo input shaft spins easily using your fingers.
As Brent mentioned usually the problems with the speedo not up-shifting are with adapter...I added a few more common possibilities.