erm...bluewind bro, actually what kind of plant are we talking about here? stem plants or foreground plants?
im assuming you're talking about stem plants. well, first of all, do u use tweezers? your problem sounds like my very first problem when i started planted tank. because i plant them using finger. huhuhu~!! seriously, tweezer does the job very well.
if you already using tweezers, for stem plants just bury them deep about 1-2 inches in the soil.
if you already bury them deep, may i know what's the size or what kind of soil/gravel you use?
if you already use at least an aqua soil (instead of commercial fish-only-tank gravel/sand), then check your fishes/shrimps. is there any fauna you;ve kept in there that actually IS NOT suitable for planted tank? (eg: normal black pleco - they swish their tails damn hard and can make plants flew off).
the bottom part of stem plants getting brownish or worse, melting is showing that the lights isnt strong enuf to get to that part. but IME, if a stem plants get bushy, normally the bottom part that near to soil arent looking so good sometimes... but heck, they're bushy and the bototm part isnt visible anyway. the plant still thrive because the top part is receiving lights still and the roots still digging (providing your soil is rich with nutrients or at least ADA).
hope this helps and if there any wrong someone please help me correct it. thanks...
Yeah, was talking about stem plants.. I don't have tweezers and I just use my fingers to plant them.
If I can recall correctly, the gravel I used was the brownish type and the size was probably medium but a bit smaller? Not too fine or too rough.. And maybe not enough CO2 inside the tank?