It sounds like you might need to adjust the clutch cancel switch if you think its related to how far down you press the clutch. If that seems OK, then you can pull the starter and have a look at the contacts inside the solenoid.

Usually if its worn solenoid contacts, you'll hear the loud clicking of the solenoid plunger pulling in but not making contact, vs. the quieter clicking of the circuit opening (or some other internal) relay clicking on when you turn the key.

In my case, I had the latter case (contacts were replaced a few years ago and still in good shape), I added a relay between the ignition switch and solenoid and now the starter cranks every time I turn the key.