Clutch pedal looses compression and gear wont change until restart the engine
 

I own a Mazda 1.4 Antares diesel manual car. while shifting gear from 2nd to 1st on an uphill , it stayed in 1st gear & not moving elsewhere. On switching off and restarting engine it went alright and didnt show any problems.Contacted local mechanic and mazda service centre, they could not find any fluid leakage or external problems. Same problem with other gears happened again two times while changing gears from 2nd to 3rd and from 3rd to 2nd. Felt clutch pedal looses its compression partially and regains compression after restarting engine. please help me in this regard by posting your suggestions. Thanks everybody in advance.