Mapping Problem

I am hoping some BizTalk Guru out there will be able to help me out with a fairly rudimentary problem I am having with BizTalk Server 2004.
It is fairly early days in project development but I have run into a problem that I am not able to overcome.
The scenario is:
I have created a project with a few schemas and a map. I then created the send and receive ports and location. All good. When I put a file in the receive folder it gets moved to the send folder. Now, if I alter my map in anyway, this change does not take effect when I redeploy the application. For example if I change the destination schema in the map this new schema does not get picked up.

I tried to be really cunning by creating an entirely new map within the project and using this in my receive port. All this looks good (ie I can add it successfully to the receive port) but then when I put a file in the receive location, it disappears and does not appear in the send folder. In the HAT tool, the error message that is given is \”Invalid pointer\”, so I guess it is never seeing the new map.
Do you have any idea where the problem could lie?
Any help would be most appreciated.
Thanks,
Caroline