Sometimes with the Duo-Packs the first amount of epoxy that comes out of the tubes is not mixed correctly. You may have too much or too little of one part or another.
We generally recommend that you expel about a dime's size worth of the epoxy out of the Duo-Pack and discard it. That should be enough to have each part properly mixing as it comes out of the Duo-Pack. The rest of your joints should cure correctly.