Bike won't start if parked uphill

[Slagugglan]

Lately i have been noticing some peculiar behaviour on my thumper: when i park it on a slope, like uphill, it is hard to start.
Usually i crank it for about half a second: two revs and the little 250 starts as fine as can be... But after being parked on a slope it just won't start. I have tried with PRI and putting it in reserve, it just won't start. What I do is I push the bike to a flat spot, wait two seconds and I start it.

It's not like it's anything dangerous, just strange. I wonder if anyone has noticed this or can come with a reasonable explanation!

Thanks and ride safe!