Forum:Mission glitch

Iv'e got all the current dlc and am level 61. I just finished the game on the second playthrough and then realised that although i had already completed the buy a shield mission from dr. zed, i had to complete it again. So i went back to fyrestone and bought a shield but couldn't turn it in because by then dr.zed has gone. This also happened with the mission where you have to talk to tannis in crimson fastness. Anybody who could explain why this is happening and how to solve it would be very helpful. Maybe its a glitch or something. If you want to get in touch my Psn's Riptide180. I also have weapons to trade =).