I might be able to identify and solve your problem, if it's what I think it might be.
You need to pay very close attention and carefully watch your arrow as it comes back during the cranking operation. I've had and seen this problem numerous times before. The only difference from the way you're describing it, is that it almost always takes place when you are within about 1 1/2 to 2 inches of being seated in the "full cock" position. What was taking place is that as your arrows nock reaches the front edge of your Weaver Rail, the under side of the rail is beveled or angled so that when the nocks touch the rail, they are forced underneath it until they drop into the seated position. As your nocks spread open more from shooting them, the top edge of the nock catches the underside of your rail and is forced forward off your crossbows string as you are cranking the cable back. This obviously dislodges the arrow from its cable position and forces you to let down the cable and attempt another load.
The solution, if this is the problem that is happening on your crossbow is two steps. I would suggest taking a grinding wheel on a dremmel tool and turn your TAC15
upside down on its back. Then grind the edge off the under side of the Weaver rail to remove any edge and possibly create more of an angle to allow a smoother transition.
Secondly, replace the PSE nocks with "Firenock D's, since they are a much stiffer synthetic material and don't have a tendency to spread as easily as the PSE nocks. They are also cheaper by the dozen than the PSE nocks.
Last note, If you have reviewed my article on getting rid of your Whisker Biscuit, you'd know that more and more TAC15 owners have been replacing their Whisker Biscuits with a QAD Drop Away Arrow rest and also cutting 2.75" off their Weaver Rail. This totally eliminates your nocks or vanes from ever having any interference with your scopes rail or anything else on your crossbow.
One of our other very highly respected field testers is in the process of having a brand new QAD Drop Away Bracket machined to his high level specifications and is also having his machinist make the modification to his Weaver rail, so the angle is perfect and he will maintain a perfect finish to his powder coating.
Either way, he now understands the performance advantages derived from performing these modifications. If you are interested, he may be producing several of these arrow rests and might offer them for sale. One of the big benefits is that a shooter is no longer locked into using four fletch arrows, so you can not only use standard three vane arrows, but you can also maintenance your arrows with any standard fletching jig.
Hope this helps,