By any chance are you using an HHA Optimizer Speed Dial on your TAC15
If you are, I had the same problem with mine and I found out that most of the time it's related to a problem with the HHA Optimizer Mounting Bracket.
There is an older style bracket that does not follow the Weaver Style or Picannini rail on the top of the TAC15 uniformly. In almost all cases it causes the problem of running out of windage adjustment at 20 yards during initial sight-in since that's the first distance the directions are asking you to zero in at. It doesn't matter when you bought the HHA Optimizer because you would have no way of knowing how long it sat on the sellers shelves before you purchased it, so the best way to tell if it's the older style bracket is by turning the HHA Optimizer Bracket upside down so you can see the bottom of it. On the side, where the two bracket tightening screws are located, if you look between the housing and the clamp portion, if you can see two set screws, then it's definitely the old style bracket. Contact the manufacturer and inform them of the problem you are having and what type of crossbow you are mounting the Optimizer on. They have been absolutely outstanding at replacing the brackets with the new ones and they respond very quickly.
The way you can tell if your problem is related to the Optimizer is by simply removing your scope from the HHA Optimizer and then mounting it directly on the Crossbows Mounting Rail. You should then be able to zero your scope in and re-center your windage at 20 yards.
If you are not using an HHA Optimizer Speed Dial, then I would suggest you remove your scope mounts and recheck how you have them attached to your Weaver Rail System to insure the scope mounts are aligning correctly.