A couple additional comments.
First of all, I notice that the 1st post says that the problem is tracking the arc, and not that you can't get better reception on a single sat, like the south sat?
A bigger dish will have a narrower beam width, and you need to be more accurate in your alignment. The smaller dish may not have really been tracking the arc, it may have just been that the wider beamwidth made you THINK it was tracking the arc.
Also, the 1st post seems to suggest that you didn't go through the alignment process with the new dish, but just expected the old settings to be the same. SatAV is correct that you shouldn't need to change the motor elevation, provided that it was set correctly in the first place, but many don't set the motor elevation properly, and with a smaller dish the error may not be noticed.
Also, as someone mentioned above, Fortec dishes seem to be of rather poor quality with respect to the lnbf being at the actual focal point. On my Fortec dish, I found that the focal point was about 2" away from where the LNBF holder put the LNBF. This could end up having you mis-align the old dish with respect to both declination AND with respect to the apparent azimuth, ie mount on pole aiming south. Again, if you ended upchosing the best compromise between south and extreme sats, the smaller dish may not have noticed the difference.
Ie you really should have started over and align the new dish from scratch. If you were VERY careful with the motor elevation, and used the modified tables, then you can leave that the same, but I'd recommend not trusting any other setting, particularly the azimuth adjustment of rotating the whole mount on the pole.
If it still can't get good results, then I'd suggest that you try an experiment that I did a couple times, by putting a bunch of little mirrors on the dish surface, and aim the dish at the sun, to see if the spots focus on the mouth of your lnbf. Like I said, when I did this, the true focal point was off by 2" on my Fortec, and I know of a fellow over on the Sadoun site whose Fortec's lnbf holder was off by about 8". Since these WS dishes have 3 arms, they are not likely to be off, but if your alignment was affected by the original Fortec dish being off, then you can't expect to just put the new dish on, and expect it to work.