This is basically lower clutch cylinder problem.its seal leakage cause to drop brake oil fluid, whenever clutch pedal is pressed ,fluid pressure not hold inside cylinder.
When fluid pressure drops so clyinder pin not pushed with force to change any gear.
Better to replace slave or lower clutch cylinder.
Dnt try to repair it.
And new clyinder must be of brass material from inside,its pistion,spring etc...otherwise plastic made be creat issues soon.