Leaf curl in Pachystachys (houseplant)

I have a few Pachystachys (Golden Shrimp, Lollipop) plants growing indoors, which I have had for several years. The leaves on them always seem to be curled/twisted and quite brittle. Other than that, the plants seem healthy, and actually are in flower most of the time. I'm trying to figure some kind of diagnosis. I water them lightly and frequently with a quite weak but acidifying fertilizer solution, and they seem to like this, having taken on a dark green foliage during the few years I have used this watering scheme. I cannot find any insect life on them, at least not for the last few years. They are growing in a coolish room (50-65 deg F) most of the winter, with a southern exposure. They show no signs of leaf burn. The room is quite dry, relative humidity probably typically in the 30 percent range.
Any ideas on causes for the leaf curl? Can't seem to Google up much info on this plant and its disease issues.
Thanks, =-=-=-=-=-=-=-=-=-==-=-=-Mike Lacy, Ft Collins CO 80523 Clean out the 'junk' to email me.
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