Simple electrolysis question

[Rockjob]

I found this website the other day. I'm very impressed. Also inspired to replicate some of the projects.

The question:
All the theory I've found on electrolysis states that oxgyen forms at the anode and hydrogen at the cathode.
In my experiments and in the oupower electrolysis basics section, bubbles are only seen to form on one of the electrodes.
(I used NaCl as an electrolyte.)

What is the cause of the bubbles only forming on one electrode? Am I doing something wrong?

I've googled this question a few times and found no answers.

[Lightwave]