There's at least two plants going by baby tears: H. micranthemoides and M. umbrosum. The former seems much pickier: mine in my 8 gal keep dropping older leaves like yours does, but in my .5g nano I've gotten a nice little bush of it. I planted them in groups of three in both tanks, but the best growth has been from stems I laid down to propogate. My best advice is to be patient... it took a couple months for the ~4" stems in my nano to grow bushy. Its listed by many as a fast grower, but it doesn't grow fast at all for me. I think its wisest to put it near the front of your tank where it will only be partially shaded, but the unshaded parts might creep like a carpet.
M. umbrosum on the other hand seems to grow everywhere, and if lower leaves are dropping I would suspect nutrient deficiency, or if its new, that maybe you started with emersed growthor something. M. umbrosum grows pretty fast even in my low/med light set-up.
HTH
