When you depress the clutch pedal,it should actually separate the clutch plate from the flywheel, so allowing smooth gear changes. In your case this is not happening, and could be due to a number of faults. Firstly I would check the clutch master cylinder for leaks (check the reservoir to see if the level has fallen). If that's ok then check the clutch slave cylinder again for leaks, and replace if necessary. If none of the checks reveal any leaks or faults, then I'm afraid it may require the the clutch itself to be stripped and checked, ie the pressure plate, release bearing, and friction plate. Unfortunately, if the problem only happens now and again, it would be nearly impossible for a garage to identify the problem, without stripping the clutch mechanism down.