Dust on dish ! low signal ?

Dry, non-conductive dust won't have much affect.
A damp layer of resistive dust (e.g. carbon coal dust or salty sand), would absorb the signals.  Wash it off, while the transmitter is turned off for your safety.

Check the feed radome window also.  It is important to keep this clean and avoid conductive salty water films.  The transmitter MUST be off when this is cleaned.  Some feed windows are pre-treated with a coating to repel water.  Distinct, separated, water drops stuck to the window are preferred to continuous water films.

Most degradations are due to bad F connectors, corroded connectors, moisture/water in cable or feed and dish movement (mis-pointing)

Best regards, Eric.