I dont think there was a particular skill issue that accounts for Wasim's lesser impressive numbers.
He has a peak of nearly 50 tests averaging 20 with a WPM of 5, comparably to anything from Ambrose and McGrath.
stats.espncricinfo.com
There were three issues:
- Debuting very early. Pacers normally debut early 20s and take around 10-15 tests to hit their strides, he debuted 18 and took 30 tests to do so.
- Diabetes around 98. Wasim had achieved a level of skill mastery but diabetes reduced his pace a couple years earlier or so than normal aging, allowing batsmen to play him out of respect without losing their wicket. This became clear to me when Australia toured Pakistan in 98 and Wasim was bowling excellently, yet for some reason the batsmen survived. The Aussies explained later that their strategy towards Wasim was to just limit his damage, and without that extra sting of pace, his swing was enough to keep best batsmen quiet but not to run through a lineup.
Imagine this same spell with an extra 2-3 MPH.
- Slips. Wasim was a fantastic new ball bowler, not far from McGrath and Ambrose, but if your fielders are regularly shelling 1-2 catches, that is bound to impact your numbers.