I'm assuming your mounting post is perfectly plumb. If so, most likely you are having problems with skew. You didn't specify which orbital location you centered the T90 on, but there may be an easier and better way of doing this.
Because the T90 can cover about 40 degrees in orbital locations and 129W is the farthest west you can go anyway on Ku, you might consider centering on 110W. That will leave some possibilities of adding more orbital locations to the east in the future, and should make your current alignment easier. As SatLex doesn't give very accurate positions for LNBFs on the T90, I've found it's often better to simply ignore it.
If you choose to do this, put the 110W LNBF exactly centered on the T90 arm. For Dallas, your T90 elevation for 110W will be 49.3 degrees and the true azimuth is 203.4 degrees (I didn't bother looking up your magnetic declination). The T90 skew should be set to 19.5 degrees (west side of dish lower than east).
Once you have 110W peaked for maximum SQ. Put the 119W LNBF at 9-10 units on the T90 LNBF bar (east side). If your skew is set in the ballpark, you should be able to tune the LNBF holder position for maximum signal by moving it along the bar.
Now nudge the 119W LNBF mouth up and down slightly to see if the signal increases in either direction. If not your skew is decent. Otherwise you will need to crack the skew down or up on that side. It works kind of backwards as I recall - if the LNBF gets a better signal when nudged up, you'll want to rotate the skew so that LNBF goes down. Such is the fun of a dual reflector. Once you get the skew peaked on 119W, you may want to retweak its position, and in particular make sure 110W is still peaked. If not you will have to iterate, but don't change the 110W position on the LNBF bar.
After you're happy you can do the same for 123W and 129W. I would suggest putting on the 123W LNBF first, but not adjusting it because 129W has a lot more output level. Instead put the 129W LNBF somewhere around 19-21 units east of center and repeat the procedure for it that I outlined for 119W. Then do 123W. Hopefully you'll be done messing with the skew. 97W should also work pretty well.
In the end you may still need to do some tweaking overall. One other variant would be to put the 97W LNBF first on 89W or 91W after doing the 129W alignment to make sure both extremes of the T90 arc are correct. That makes adding LNBFs much easier in the future, and all but guarantees 97W and 123W can be found merely by moving the holder back and forth on the rail.
Aligning a T90 is very easy, but only when you get the hang of it.
I've been researching and stressing for the past week for hours and hours. I'm sorry you guys! Does this forum accept donations?
