What am I doing wrong ?

This has been an ongoing problem for me, so I hope someone here has an
answer, every tank I have set-up or ran in the past until now (except for
one) I can not get any plants to grow, nor live past 2-3 weeks. I have done
all what the LFS said to do, I have bought several different bottle of plant
food drops, and still they die.

What could I be missing out on for any plant to die after 2-3 weeks.

I have the usual growlux type fluro on about 8-10 hours a day, regular water
changes, plant food drops, different tanks have different substrate, and of
course fish.

-Andrew