[Dave at Pelican Parts]

Nick, I think your reply got attached to the wrong question...

As an attempt at an answer:
Problems with all gears tend to be clutch related. If you are having trouble getting the car into any gear at all when stopped, the clutch is the primary suspect. Note that the speed difference between the engine RPM and the (zero) wheel RPM will be smaller in higher gears, so they should be easier than the lower ones. But a problem across all gears is most likely to be a clutch problem. Either the clutch plate, or the hydraulics.

Another possibility is that the shift linkage has gone wonky. This may be easier to check than the clutch--inspect the cables that actuate the gear change and make sure they are routed correctly and not sticking or kinked or getting hung up on anything. This could potentially cause issues with just a few gears; usually those gears will have something in common in the gear shift pattern. Like all the ones on the left will have trouble, or all the ones that you push the stick forward to select, and so on.

Problems with a single gear are more likely to be inside the transmission itself. You may be able to improve those by changing the transmission fluid, but that is something of a bandaid. To properly fix problems inside the transmission, a rebuild is generally needed. Hopefully it's not that, or at least not that serious.

Good luck!

--DD