What type of meter are you using? The cheaper ones (SF-95 and similar) will show you only the strongest signal.
It's possible that the dish and/or lnbf are just enough out of alignment that the weaker transponders can't be locked onto. Use you sat receiver's signal meter to peak the dish on one of the weaker transponders (lower signal level/quality) you are currently locking onto. As you're already pointing very close to the sat, you shouldn't have to move things far at all. Here's what I would try doing, but someone else may have a better suggestion.
1) Have you already skewed the dish? The site I provided a link for will previously give you the proper skew once you have entered the east and west sats you wish to pickup. Hint: To force 97W to be your "center sat", choose east and west sats which are equal distances from 97W. For example: 77W and 117W for a 40 degree spread centered on 97W.
2) Aim the center LNBF in it's holder left and right to peak the signal level/quality. As the center LNBF, is should be at 0 degrees. If not, then the dish is off L/R and you will need to adjust it or move the LNBF down the guidebar a little.
3) Adjust the center LNBF's skew in it's holder to peak the signal level/quality. As the center LNBF, it SHOULD be at 0 degrees skew assuming the dish is already skewed correctly...if not, then the dish isn't skewed correctly and you will have trouble with getting the other sats
4) Repeat 2 and 3 as necessary to obtain the best signal level/quality.
After peaking the signal as above, if you are still having troubles receiving some of the transponders, your dish may not be pointing correctly for either L/R or elevation...something these dishes are quite sensitive to. I discovered yesterday that my t55 is currently slightly off-center (L/R) due to the clamp being a little loose and getting knocked around. As a result, I'm experience similar issues: Not all transponders are locking well, and I've even picked up 95W and 97W at the same time--scanned in the CCTV channels from 95W but couldn't get all of the 97W channels (mine being a smaller dish sure doesn't help). Hopefully the elevation's not off...the t55 is heavy enough, I can't imagine adjusting the t90 elevation easily unless the mount has a different adjustment method (maybe a screw/turnbuckle?).
Good work so far! You're close, so it shouldn't take much to get them all...