This is an interesting one. Defence strengthening often alludes to the frailties of the four defenders and a goalkeeper. One or more may have weaknesses.
However, goals are often scored because of a frailty or weakness in front of those players. The shape is lost drawing a defender out of position or resulting in the defence being out numbered.
It could be concentration at certain critical points, it could be tactics at certain critical points.
We certainly seem to have had a problem for a number of years now that we don't defend leads well. The fact that, players, managers and coaching staff have all changed so much means a few individuals can't be responsible for that problem.
So perhaps the lack of continuity is the problem. People need to learn their trade, learn how their colleagues operate. Who responds to praise, who responds to a kick up the arse, who is best at what in certain situations.
Perhaps we need to invest in time, patience and good coaching.