Usually, this issue can be fixed by simply repositioning any loose hangers and making sure all others are sturdy in place. This should straighten out any problem areas and correct any sagging. If this quick fix doesn't resolve your issue, you may need to replace your hangers for extra support.

Firstly scrape out any dirt and debris from the gutter joint, and make sure it is dry. Then just apply some gutter sealant to the joint, and squeeze the gutter pieces together to ensure a strong connection. If your guttering is connected with a union clip, this may need replacing if it is broken or damaged.