I've had a similar problem with 2 of the 3 kit cars I have. I went round the car body and marked all the blisters with a triangle of masking tape. Here's a picture of one of them after I'd removed the gel coat.
You can see that there has been an air pocket under the gel coat and the fibreglass looks dry rather than having been covered with resin.
Best located with a heat gun or a black paint coat and some time in the sun so that it can be sorted before final paint.