NT file lock notification problems

I have two domains in two different locations. There is a particular folder which is used by our sales support group. This folder contains an excel file which is in constant use between the two domains. I have a complete trust relationship between the two domains and 2 WINS servers, one in each location.The one in location A acting as primary and the one in location B acting as as secondary and vice versa. A user in location A will open the excel file and make changes to it. A user in location B will open the same file and she will get a notification that she has read access because the file is already in use which is what shuold be the case. Before user A saves her changes, B will be notified that she now has write access. She wil start making changes to it. When A finishes and tries to save her changes she will receive a message "file save not complete. Retry or cancell". Eventually nobody is able to save. The problem normally happens between location A and B and neither in A nor B.Can somebody help me??????