If the trick is not observed:
This photo shows how the outside thread is skipped if the trick is not observed when weaving twill.
Here is the trick:
Always start the twill weaving sequence by entering the shuttle on the even side of the warp. The “even side” refers to the side where the outside thread is threaded in a heddle on an even shaft. For a 4-shaft twill, that would be shaft #4 or shaft #2.
If you didn’t follow the trick:
If you are weaving along, and you notice the problem, you won’t want to interrupt the sequence of the twill weave. In this situation, you can cut the weft thread and enter the shuttle on the other side of the warp. In other words, if the shuttle was on the right edge of the warp and normally would be entered from that side, cut the weft and enter it from the left side. This procedure might need to be done a couple of times to get the right sequence and side of the warp to work out.
Of course, you can take the shuttle around the outside warp by hand as you enter the shed. To do this, you’ll take the shuttle around the outside warp thread and place it into the new shed. You can do it every few wefts—just to keep the outside thread woven into the cloth. Another option is to use a floating selvedge. This is the solution I would recommend if the twill changes direction (zig zags).