Ever since some issue between me and my husband our relationship turn plain and sour. Don't get me wrong, he is still trying his best and he didn't have another party too. Wonder is it normal for relationship to turn plain?

Yes, I agree that it is common for relationship to reach a status quo. While it could be some lingering “remnants” of the issue still clouding over your relationship, it could also just be a lack of new activities to excite the both of you. Since it sounds like both of you are trying to move on from the issue, why not dedicate a fresh start. How about taking a staycation/vacation or picking up a new hobby together and try to have some changes in the relationship. This may make it easier to move forward I feel. Have some quality time together. If needed, thrash things out for the last time and try to move on once and for all. Take care!