Cynclair, I think the problem is due to the falling sprite lump names being capital letters.
Falling sprite lumps use lower-case letters as such: PLAYa1, PLAYa2a8, ect.
Standing sprite lumps use capital letters. PLAYA1, PLAYA2A8, ect.
If you're using XWE, I suggest you go download the most recent version, as older versions tend to rename lump names to capital letters by default.
At least, that's what I heard.
Hope that helps.