I had the same problem and had to contact Disney's support team to figure out what to do. This may help you.
First thing you need to do is make sure you have a registered account with Disney and the HandsOn network. You can do this by going to the website and signing in under "Manage Accounts". It should list out yourself and all other volunteers that are registered.
If it says that it does not have an account set up for you, you probably didnt take the registration process originally all the way to completion. You just need to go in and register taking the process all the way to completion. It may even help to sign up for a project in the future (that has a set date and looking for a specific number of volunteers) so that you can confirm you are registered with both Disney and HandsOn. If you do that, just make sure to cancel that sign up if you dont plan on going.
The support individuals state that it takes about 24 hours for new accounts to be set up in their system (with todays software, that seems kind of silly but it is what it is). Once they have you in the system, they can contact the HandsOn network on your behalf and ask for confirmation that you did indeed complete the volunteer work. This seems to be much more effective than contacting the volunteer group yourself (who are probably overwhelmed with volume). If the group confirms your attendance, than HandsOn will verify that in the system and Disney will issue you a voucher. You should get an email stating that all this has taken place or you can just check your account on the website.
I hope this helps.
