Good Afternoon

I hope you may be able to offer some comments on my lack of success with my
balcony garden.
For nearly two years I have tried growing simple annuals in long rectangular
plastic pots. I have used everything from cheap, to good trade name potting
soil.
Sunshine, good food, water, nothing makes any difference.
I buy strong, healthy seedlings from K-Mart but half don't grow and those
that do, offer four or five blooms then just seem to give up.
I have bought many dozens of plants, in about six different varieties, all
quite simple.
I have noticed that, when in despair, after a couple of months I pull them
out, the roots have just not grown at all.
What am I doing that is wrong ?

Would appreciate your comments.

Kind Regards

Kate (Sydney, Australia)