What can cause an inlet valve to get stuck?

[Micha]

I found the intake valve stuck in the open position, about 2-3 mm. Took the head apart and had to hammer the valve out from its guide with a piece of hardwood. There were shavings from the brass guide visible.
The whole area was well oiled. The push road is delivering plenty of oil too.
Took it back to the workshop that overhauled this engine 2 years ago (I supplied them back then with new valves, guides, springs etc, quality stuff made by www.intervalves-technologies.com).
I had 1 guide spare and they used it now and managed to fix the head.
They also confirmed that it was not a poor oil supply. On the other hand, they couldn't say what caused this malfunction and that's actually what interests me the most here in this whole story.
I had a hypothesis that maybe they didn't make a precise enough machining at the time, and that the bore for the valve stem was too tight (which they don't want to admit).

What can cause an inlet valve to get stuck?
Since that overhaul ca. 2 years ago, the bike did only 100-150 miles.

[Seek]

Fit too tight. They need to be looser then you would say at first sight. I haven't measured my newly refurbished valve guides yet, but I can surely feel some wobble.

Or the valve stem was not straight.

[Micha]

Fit too tight. They need to be looser then you would say at first sight. I haven't measured my newly refurbished valve guides yet, but I can surely feel some wobble. Do you mean some minor side play?

Or the valve stem was not straight.

[Seek]

Yes

[vechorik1373]

I agree. Too tight of clearance between the valve stem and the guide. Make sure that now used valve stem is straight, and does not have any gall marks on the stem from seizing. If it does, replace it.