Thanks to all who have responded. I immediately replaced the fan with
another one - had to devise a way to mount it on the video card heat
sink which wasn't straightforward. I was just curious about why the fan
was so position sensitive.

Bennett Price wrote:
I've got a small 12V DC .9 Watt 'muffin' fan (1.75"x1.75"x3/8").
It was the cooler for a video card's processor chip and was
frozen up.

I put a drop of oil into its sleeve bearing and it now runs
reliably - almost. It will start every time as long as it is
not oriented with the open 'face' down. (The open face is the
side in which you can see the bearing). If the open bearing is
down, it sometimes starts and sometimes needs a slight nudge to
get it going.

What's going on? How come? TIA