Only beers recently that I've struggled to maintain head in are a coconut porter that has a reasonalbe amount of visible coconut fat in each bottle and two beers (one pale, one dark) that separately carmelise a small portion of the wort during the boil. I have caramelised previously with no issue and they do seem to be improving (especially the pale) but 90% of retention issues in my hose coincidentally have a caramel portion so I figure there is some relationship - maybe the degree or caramelisation?
Hop oils will help so maybe try a small portion of late hops (I realise the beers you describe are not meant to be particularly hoppy).
A short 55 rest and a short 72 rest seem to give most of my beers, bar those described above, great retention and formation (sometimes too much).
Presuming you've been through all the usual crap - clean, well rinsed, residue free glass, different temps etc.
If carapils is working, with all else being equal, then it suggests dextrins are absent. However, I would have expected a 71 rest to accomodate this, depending on how long the lower sacch rest was. Maybe shift the balance between low sacch and high sacch/dextrin in favour of the dextrin.
The next bit is taken from my reading rather than direct experience so take with a grain of salt:
Zinc will also contribute to foam stability - do you use nutrient or other zinc additions?
Don't believe it relates to head retention at all but sulphate and dark beers are not recommended by some. I use chloride only with my stouts and porters.
Some proteins that contribute to haze may also contribute to foam stability so maybe try inclusion of some cold break if you are separating. Those beers don't need to be served super cold anyway.
Higher protein malts and not using adjunct/lots of adjuncts should theoretically lead to better foam stability.
How long are you boiling for? Long boil times (not sure how long but presumably a fair bit longer than the average 60-90 minutes) may contribute to the breakdown of some hop acids that affect foam stability.