I think it's a 2 step process
First you buy add-on and fund get transfer from your credit card to public mobile available funds then you buy addon again from available funds.
Login to your self serve and check your available funds, if you see extra fund in your account then buy again add-on from available funds. Also reboot after you purchase.
If you followed the process and still doesn't work
Then send a msg to mod
Send your email address
Phone number
SIM number
Explanation of issues
http://productioncommunity.publicmobile.ca/t5/Getting-Started/Private-Messages-viewing-and-creating/...Hopefully they will get back to you soon. Thx