Moribund plecos

I notice I have a really high death rate of plecos. I keep cichlids,
usually moderatley aggressive south americans, haps and peacocks. I have
several tanks. I try to keep at least one pleco in each tank to eat the
algae. I have a problem though in that they are always dying mysteriously,
particularly the small ones. I keep enough of an eye on them to know it's
not ich and the cichlids pretty much leave them alone, so it's not
persecution or being eaten or ripped apart (at least, very rarely)

They always seem to survive once they get about 4" -5" long, but until that
point, it seems like about a 60% death rate, compared to maybe 2% for other
new fish. What give here? Are they starving? Eating too many toxins?
Don't like water quality, hardnesss, temperature?

Any advice would be appreciated.

BD