I've actually had no difficulty in inheriting my PUs in the current game (well, partly because I reloaded a couple of times if ruler death occurred mid war - what can I say, life's too short).
For me, it's low infamy (Mine is always below 5, usually only a point or 3), high relations (175-200), & time that seem important - no one knows of course. I inherited 18 in 1581, 15 in 1595 & 19 nations in 1641. Never had a sniff prior to 1581. The most recent inheritance was with a diplo 3 heir inheriting. My SOI has been BIG at all times, too.
I suspect size may matter. I'm far bigger than anyone else (cough).
I can't make out quite what is is that causes inheritance. For me, I know that if I just wait long enough, keep PU relations up, infamy down, I will eventually inherit PUs.
When I do inherit, what I have noticed is that it tends to be in great swathes of nations. Those I inherit tend to be (mostly) bunched close to each other :
So, in 1581 it was Albania, Auvergne, Brittany, Byzantium, Corsica, Croatia, Denmark, Gotland, Guyenne, Holstein, Hungary, Lithuania, Mazovia, Moldavia, Norway, Poland, Ragusa & Sweden.
In 1595 Augsburg, Austria, Baden, Bavaria, Bohemia, Bosnia, Brandenburg, Brunswick, Cleves, Cyprus, Frankfurt, Hesse, Lorraine, Luneburg, Saxe-Lauenburg (most of central Europe).
In 1641 Aachen, Aquileia, Baden, Brabant, Flanders, Frankfurt, Genoa, Magdeburg, Mantua, Modena, Montenegro, Naples, the Palatinate, Provence, Savoy, Sicily, Tuscany, Venice.
(In this game I've PU'd virtually all the monarchies of Europe, pretty obviously) I have yet to inherit Friesland, Holland, Murom, Muscovy, Pskov, Ryazan, Smolensk, Tver, Yaroslavl. Pskov has been in a PU since 1499, which puts the boot to the idea that time is an over-riding factor in inheriting, but then again, the natiosn I've yet to inherit are (except for Holland & Friesland) pretty much clumped, it's pretty noticeable on my map.