Why White Sauce Turns Soupy in Asparagus Pea Casserole

We have an old family recipe, English Pea & Asparagus Casserole, that we make every time we have a ham. It uses drained cans of peas & asparagus, and you layer it with sliced hard-boiled egg and then pour a white sauce \[béchamel\] over the top, and bake until it's heated through, top with a bit of grated cheddar. \[It sounds weird but everyone raves about it. I, however, have a reaction to peas, so I've never tasted it.\]

In recent years, after baking in the oven, the white sauce just returns to liquid, and it doesn't set up as it cools. So it's almost like a cream soup. My mother had been making the white sauce in the microwave. I thought perhaps something weird was happening via that process. So this time around, I made it the traditional way, on the stove top. The result was a beautiful white sauce that both my mother and I thought perfect. But still, after the casserole went into the oven, the white sauce went back to being soupy.

My next thought is to 1) try a more thorough drain of the peas and asparagus \[I don't know how thorough my mother is when doing this part\] or 2) use frozen peas \[cooked\] and fresh asparagus \[cooked\], but it will probably be months before we have another ham. Other than those two ideas, do you have any suggestions? \[I am happy to share the recipe.\]

Thank you.