There can be many causes.
Either the antenna isn't capturing sufficient signal strengh and 'quality' or it's being lost in cabling/connections.
Check your antenna hasn't moved.
Check all connections, especially at the antenna... water, rust, corrosion at the balun/terminals.
Ensure you're using good-quality coax. eg: RG6 Quadshield.
Difficult to tell without accurate signal measurements, but that gives you a few things to check for starters.