If the satellite EIRP is too low in your country or geographic area for a 1.8m dish you will need a larger dish. Find the beam coverage map for your beam - you may at the beam edge or outside it.
Since you have a 1.8m dish, I would guess that you expected it to work. Was the size suggested by a service provider ? Do 1.8m dishes nearby work satisfactorily receiving the same carrier from the same satellite ?
If a 1.8m dish is expected to work where you are then the problem is not insufficient EIRP from the satellite, but some receive problem at your dish.
In this case check:
Dish assembly. Is the rim flat to 2mm ? Use crossed fishing lines. Is the surface join at any segment joint smooth?. Is it in focus ? Check dimensions and documentation - feed support struts must be in correct holes etc. Never 'force' a dish during assembly. It should fit together delicately and perfectly without any stress.
Feed: Do you have correct type of polarisation (linear or circular)?. Is the polarisation set correctly (starting position and then adjustment angle in the case of linear) ?
Check for water in the feed, crossed waveguides, crossed gasket, poor F connectors & moisture corrosion in cable.
Is the dish pointed and peaked up ?
If you are using the modem as a signal detector check the programmed tuning and symbol rate.
If C band, there is a possibility of interference from local terrestrial communications.
