My interleaving linked lists function isn't changing the lists required

Question

Im trying to interleave two lists such that:

list1 starts as {a1,a2,a3,a4,a5} list2 starts as {b1,b2,b3,b4,b5}

And I want them to be {a1,b2,a3,b4,a5} {b1,a2,b3