I had some browning & die-back on the lowest leaves on several new semps, and I finally realized it was mostly happening in a couple of containers where I hadn't mulched with sand. I think the lower leaves were just getting too wet (we were having rain that week) from contact with the potting mix, even though it was a fast-draining mix (half builder's sand, half pro-mix). So, you might try a quarter inch of builder's sand (not play sand, you want coarse brown sand or tiny gravel) and see if that helps. I also water mine very lightly any time I pass with the hose, and they're rooted in well now.
I also put the most stressed plants out of direct sun, and they're doing better.
See? :-)
