I believe there are two reasons for getting those air bubbles, insufficient pressure in that particular area and not letting the lams gas off. Check the gap in your form in the area where the bubbles occurred if it is larger that the length of the gap between the rest of the form this could be the issue, and like Kenny said I always let my limb sit in the form for about an hour before putting into the oven, haven't had any issues with air bubbles since doing this practice knock on wood.