It is possible that you have a bad sidelobe pointed at the adjacent satellite. You may also have low on-axis transmit gain meaning your transmit power (BUC watts) is much higher than it needs to be.
You need to plot the antenna radiation pattern, which should look something like this:
It is most likely that your actual patterns are greatly different, with serious imbalance between the sidelobes on either side and with the main beam much broader and lower in gain than it should be.
Possible reasons are panel distortion/mis-alignment and feed location and orientation wrong, and subreflector also if you have one.
Get hold of the manufacturers installation/alignment instructions and mechanically set up your dish using physical measurements.
Check that the rim of the dish, at its operational elevation angle, is flat. See: Satellite dish panel adjustment
If your dish was supposed to have special spacers inserted between the steel backing structue and the dish then putting the correct spacers in the right places is really important. Wrong or missing spacers or wrong positions can result in the dish being 10 dB low in gain !
Check the feed is central and pointed towards the centre of the dish (circular dish) or very slightly above centre in case of an offset dish. If you have a short f/d circular front fed dish the location of the feed is extremely critical. If you have a subreflector then its position and orientation is critical also. See: Sub reflector adjustment
. Long f/d offset dishes are more tolerant of feed location errors.
This Prodelin 3.8m ABCD measurements
link may be useful if your antenna is the same as that referred to.
Make receive sidelobe patterns yourself using a CW satellite beacon. You need a PLL LNB, spectrum analyser with 1kHz resolution bandwiodth and a plotter.
Repeat the sidelobe measurements and adjustments till you have a clean pattern with equal height (+/- 0.1 dB is worth trying for) first sidelobes and deep nulls. This could take anything from a few hours to as long a week to complete. See the sidelobe pattern image above.
It is quite common for multi panel dishes to have an initial performance 3 to 6 dB lower than designed, equivalent to a dish half or quarter the area, so don't be suprised if you get a big improvement.
Measuring the relative G/T before and after gives you an idea of the gain improvement (measure beacon level versus noise level off satellite). Looking at the receive sidelobe patterns will indicate the reduction in receive interference from adjacent satellites.
It is important get the receive patterns right first as this saves a vast amount of time and money getting the transmit patterns tested and adjusted till they are acceptable.
Get your transmit sidelobe pattern measured with the assistance of the satellite operator. You pay for this. They will assign you a test frequency where you will not interfere with adjacent satellites, or will limit the angular range of the testing. Transmit a CW carrier and drive the antenna steadily +/- several deg both upwards and left to right across the satellite while the ESVA test earth station plots your elevation and azimuth sidelobe patterns.
Review these patterns and make adjustments. Repeat the process till your antenna is correctly set up. Hopefully your antenna will pass on the first test but more likely you will have to make adjustments. It may take a few hours if you have prepared your dish yourself first.
Best regards, Eric.