Hello,
Sorry to hear your car is playing up
I'm going to reel of a list of variables in no particular order.....
Did you install a full clutch kit?
Pressure plate, friction disc and bearing?
If the old pressure plate was used it maybe weak or worse broke.
This is not critcal did flywheel treated to some wet and dry sandpaper to remove any glazing.
Could the clutch friction plate be in backwards?
Did the gearbox get a new oil seal on the transfer shaft (not sure of its proper name)?
If not it maybe its leaking oil, possible that the friction material is contaminated?
The clutch mechanism actuator arm, spring (clip type of thing) and so called 'top hat' were they replaced for new?
I would remove the gearbox and work backwards checking each part and replacing any worn or defective items.
The car gears worked before the gearbox swap?
This may indicate the linkage to the gearbox and the slave cylinder may well not be at fault.
Good luck
Edit.
Even though a clutch is a fairly simple mechanism there are a lot of other system variables to consider when they malfunction.