Is there a known rule for Latin and Greek *p and *kʷ - in other languages

[Anixx]

It seems to me that some words that have -p- in stem in Latin have clearly reconstructible -k^u̯- based on other Indo-European languages. Some examples include

- *u̯lpos - *u̯lk^u̯os ("wolf")
- *u̯esper - *u̯esk^u̯er ("evening")
- *apa - *ak^u̯a ("water")
- *i̯epr - *i̯ek^u̯r ("liver")

The "k^u̯" reconstruction is supported by the majority of languages, still many authors consider the roots with "p" as genuine and/or separate. For example, Katz tryed to proove the connection between *u̯esper, Hettite *u̯esp "cloth of the dead" and Greek ospros "pulse".

Mallory & Adams reconstructed the PIE water deity as *H₂epom Nepots instead of evident *Ak^u̯am Nepot(s) (note that the laryngeal here is not supported by the Nostratic evidence).

Starostin lists both *ap- and *ak^u̯a; *welp- and *welk- (and also *lup-) as separate roots.

I wonder why the correspondence between **k^u̯* and **p* is not evident to some authors?