Question

Why do bacterial transformations fail at times?

Answer 1

Lots of reasons
I'll list a few below

Explanation:

I'll just refer to the "heat shock" process.
-perhaps your bacteria all died because you left them in the water too long
-none of your bacteria took up the bacterial resistance plasmid
-your ligation reaction didn't work, so your plasmid was linear.
-you used too much AMP on the plates

Lots of potential problems with reaction conditions, reagents, enzymes.