Bubbles are literally sealed bubbles of air under the wax.
They show that that things that are hidden or deeply buried will be brought to the surface.
In positive work this often is spirit letting us know that there is information we do not have and making sure it will be brought to our attention.
In negative work it means that hidden secrets will be revealed and they will probably be part of how the work manifests.
It often specifically shows hidden events or shame being revealed so they can either be dealt with or healed.
What To Do:
This is not a bad sign for the success of a work. There's nothing to do about it, but it would be good to ask spirit for more details or get a reading if you want to prepare for what is coming up.
Also, if you feel this might be about something you personally have been hiding, come clean as quickly as possible so you can do it on your terms, rather than waiting for the magic to bring it out.