You got to adjust the dish when both supports arms are on, if you adjust the dish first without the support arms,signal is good, if you decide to put the arms after some time your signal will gone because dish is gone slightly to other direction because of squeezing the dish.

I use the same dish 1.5 with same lnb & feedhorn and i found out that there is a much better gain without support arms,higher quality for some 10%.I think it's because those arms squeeze too much the dish or lnb arm and turn it for 2-3 mm, which is a lot...With one support arm is much better,2 eve better,but it's no better when the heavy winds starts.I don't know what should i do...