One of the common problem which most of us [including yours truly!] experience is converting the better/winning position to a full point! .
The common advice which I was told by most of my friends and well wishers is that we need to be very focused and concentrated from move 1 until we sign our score sheets!.
We sometimes tend to relax a bit after obtaining a better position and it could result in lapse of concentration.I agree to this advice.