We often encounter gutters that contain standing water or flow backward and overflow onto the house because they are not pitched properly. This can be due to improper installation, an incident that damaged the gutter, or from the house settling over time.
Most often, the best solution is to replace the sagging gutter. Removing the old gutter, re-pitching it, and putting it back up is often not a good option because the gutters are fragile and are almost always damaged during the removal process. Often, sagging gutters are old, beat up and ready to be replaced anyway.