When the motor's not running you should be able to shift the gears with no trouble.

When you got the clutch installed did that include new pilot and throwout bearings, and a refill of the clutch hydraulic system? You could be low on fluid, or there might be air in the system, or one of your bearings might be shot.

And for those who are wondering, here is the process for double clutching a downshift:

- press clutch pedal
- shift into neutral
- let go of clutch
- blip throttle
- press clutch
- shift into lower gear
- let out clutch
- get back on gas

No point doing it on a public road, on a racetrack combined with heel-n-toeing you can downshift while braking without upsetting the balance before the turn you're braking for.