Potato breeds such as red bliss, blue and purple potatoes, and fingerlings are waxy with low starch and relatively high moisture, and will water down your casserole as it cooks.
Also, canned or pre-sliced potatoes are convenient, but the preservatives used to process them make them release a lot of moisture, which can make sauces break and separate.
Yukon Gold and russet potatoes are better choices, with a lower water and high starch content that thickens the sauce in scalloped potatoes and helps it adhere to the sliced spuds.
Even when using low-moisture potatoes, be sure to pat all the slices dry with a paper towel before layering them in the dish to ensure a nice and thick sauce.