adding multiple test cases to test plan

[BBaird]

When I try to add test cases to the test plan and chose the button to add only a very samll percaentage actually get added.
After verifying that all the desired platforms for each test case is selected, I press the add/remove selected button. The screen refreshes but then only about 2 or 3 three test cases actually get added, the remainder just get the selection box cleared.
If I repeat this process over and over again I can eventually add all the test cases, but I have no idea why it will not add all of the test cases as selected.
Anyone have any ideas?