Dybo's law context

I want to ask you what is the real context of Dybo's law?

The rule can be formulated:

For Italic, Celtic and Germanic:

V:/RVaccented > V

or

VH /RVaccented > V

V - vowel ; R - Resonant {m,n,l,r,w,y} H-laryngeal

but
Latin fu:mus < *dHuh2-mó- is still with u:
Latin pu:rus < *puh2-ró- is still with u:

and seems that there are also samples of short vowels from long vowels
follows by a non-resonant

Any help?

Thanks,
Marius