Nina, welcome aboard. As you say your recipe works well for individual puds but not with the sausages.
I take that to mean you are making the toad in the hole in a larger pan and not in individual dishes.
All I can think of is that the oil is not hot enough, or there is not enough of it, when you make the toad in the hole. Could always try to make a Yorkshire pudding using the larger pan and seeing how it goes. And varying things a bit until you have it down. The ingredients are fairly inexpensive if you leave out the meat and would guess in a couple of hours of playing with it you will find a great solution.
Oh yes, and make sure the sausages are at room temp.
Anyway just what I would do.
Otherwise I have nothing to offer.
When you figure it out please let us know. Love Yorkshire pud and toad in the hole.
Again welcome to the forum.
Started this before YT posted, yep, great advice.