GBC Ultima 65 not sealing edges?

My preschool has a GBC ultima 65 that doesn’t like to seal on the edges or between pieces of paper run side-by-side. I’ve adjusted everything I can think to adjust(front/rear roller pressure, film tension, increased heat, slowing the machine as low as possible). We use 1.5 mil film, but it is usually a piece of cardstock that has pictures glued on the front. Are we just being unrealistic in our expectations or should it be able to seal that thickness? If it is capable of sealing that thickness, can you offer more suggestions?