Weak target protein bands and strong background at a top of a gel, what can be a reason for that?

Target protein band is very weak, why there is a strong signal at the top of a gel. What can be a reason? 

Some proteins may aggregate if the sample is heated in presence of SDS at 100°C for 10 minutes, as in this example. Aggregated target protein will therefore stay at the top of a gel and a signal from actual target will be weak or not present at all following development of a Western blot. 

An example of such protein is plant H+ATPase.