Hopefully this is an easy question.
I have IH4 on my zencart v1.5.1 shop. I have a main store that is in /shop/. I also have another store that is a subdomain shop.
Both of the following URLs do work in the browser (shows the image).

IH4 is working properly on the main shop and I get the hover when over the thumb
http://myshop.com/shop/bmz_cache/a/a...ge.150x150.jpg

IH4 does not show the larger image on hover on the subdomain shop
http://subdomain.myshop.com/bmz_cach...ge.150x150.jpg

Any thoughts?