If you have not already done so, you can make bleeding much easier if you replace the banjo bolt at the master cylinder line connection with a bleeder-type banjo bolt. The master cylinder area is where most of the air rises to, gets trapped, and cannot be bled out (easily) using the bleed nipple down by the slave unit.
If an inspection doesn't reveal a fluid leak, there's another common problem that mimics having air in the clutch hydraulic system. The clutch pushrod is spinning because the bearing in the pressure plate is not moving freely. This causes the pushrod to move the clutch slave piston back into its bore slightly.
So, when you ride for awhile without using the clutch, when you try to use it, you have to pump the lever a little to first move the cylinder back into position, after which it works normally. Low-pull aftermarket slave units accentuate this problem because a full lever stroke moves the slave cylinder piston a shorter distance than it does the stock unit.
You usually just need to lubricate or replace the pushrod bearing on pressure plate to cure the problem.