Does it shift into 2nd with engine off?
Or while moving, can you shift into 2nd?
If no to both then thats a transmission issue not new clutch issue
A transmission is always connected to the rear wheels, so will always be turning at rear wheel speed
If you are stopped then transmission is at 0 RPMs
The clutch disconnects engine RPMs from Transmission RPMs
The engine is connected directly to the flywheel and pressure plate, they all spin at engine RPMs, lets say 750rpm
The clutch disc is sandwiched in between flywheel and pressure plate, and its connected directly to the transmission(input shaft)
If engine is at 750rpms, and transmission is at 0rpms, then when you want to put trans into a gear, the clutch disc must slow down to 0rpms, period, it must match transmission RPMs
When you press clutch pedal down it should remove the pressure of pressure plate holding clutch disc against the flywheel, this would allow clutch disc to slow down to 0rpms and you can then put it into a gear
If you don't get full travel of the clutch pressure against pressure plate then clutch disc RUBS on flywheel and pressure plate so clutch disc can't slow down to 0rpms and you can't get trans into gear
A new clutch disc is thicker than old one, so you need FULL TRAVEL to release ALL pressure, where before you didn't with thinner clutch disc
With new clutch disc there is no margin of error
Once you are rolling transmission is no longer at 0rpms, so its often easier to shift gears because clutch disc just needs to slow down a bit or speed up a bit, it doesn't need to stop, 0rpms
The "synchros" in a transmission help RPM match the clutch disc RPMs to transmission RPMs, but these are just 4" to 5" discs, the clutch disc is 9" to 11" so it can be hard for the smaller discs to slow down or speed up the larger clutch disc