Did you allow the walls to dry for 24 hours before applying the sealer? If not, that's the issue; moisture trapped behind the sealer. Or is it possible that wall has a moisture issue that started the whole problem?
Unfortunately, there is little remedy but to sand the walls again. This time don't wash but just go over them with a damp cloth to get all the sanding dust off and let them dry for a day before moving on.
What product did you use the "seal" the walls? I use Zinser's Bull's Eye 123 which is a primer/sealer and have had no issues with it. It can be used outside, inside, with oil base or latex base paints.
Hang in there and you'll get the results you want!