You are probably on the arc, but are just a smidgen off (assuming there isn't a tree branch in the way or something?)... I had a similar problem not long ago. I solved the problem for my dish by going to G10 and just slowly tweaking and tweaking, and tweaking while on the weak tps with the tv outside.
Ice suggested this and it helped enormously. I got one of the weaker tps, (think it was 11720?) to go from something like 2-10% to 40-60%! Once I did that it brought all of the arc in better. The tricky part is holding the dish or motor just right as you tighten your bolts... Also, another thing that will help, if you have an invacom (or any other really) lnb is to figure out the sweetest spot and wire tie the lnb down. That will help with your skew a little too if you adjust it just right. Sometimes you can move the lnb in and out just a mm or two and that will make a difference.
I have an invacom quad lnb and it had a tendency to move a little (when motoring? or maybe it was birds may have helped out on that?)... so I tied it down, tweaked a lot and got it right on... If you have an sg2100 there's a place where the dish and motor meet where you can put about a thumb or finger to help hold as you adjust... tricky adjusting the bolts. I think I was out there almost 2-3 hours getting signal good, adjusting this bolt... tweak the next bolt... tweak the next bolt, adjust skew, adjust another bolt, etc. For skew, if you have a motor, the lnb should be perfect 90 degrees up/down... You should be able to put your head below the dish, look up the arm and eye it to see if it's off a little too much to the left or right while you are on g10... It won't be 90 degrees from ground, but from the dish's arm if you are on g10 while tweaking since g10 isn't your true south...