NEVER SCREW WITH A PIPE MOUTH UNLESS YOU ARE TRAINED!
There are many reasons a pipe will not speak ranging from a pinhole in a body seam to a bowed upper lip. As a voicer I start by looking the pipe over very thoroughly. Look for garbage in the wind way. Look for damage along the body seam. Some pipes sag and deform the mouth and much damage can also be done by idiots who never learn to properly use cone tuning. If the upper lip is bowed outward from pipe sag a straight edge is used to push it back into position. Even the wind pressure should be checked with an accurate and recently calibrated gauge.
Never pull or push the upper lip unless the pipe has damage and is deformed from sag. The upper lip and lower lip should have a relative position determined by the voicer of the whole rank. If the upper lip is pulled out the 3rd and 5th harmonics are increased and the pipe can sound "wooly." It will speak again but your harmonic content will be altered from the timbre of the rest of the rank.
Assuming the upper lip is properly located over the wind way and that both are parallel with the wind way and not screwed up, the pipe will not speak unless the languid is in a correct position. Voicing tools such as languid depressors, languid rods, and languid raisers are used to position the languid so that the wind sheet is just touching the outside of the upper lip. I won't go into the voicing tricks used to find the wind sheet position but yanking the upper lip out to make contact with a wind sheet that is too far out is not the answer. It is pipe damage.
Many poorly designed chests are made so that the wind produces a concussion on the lower side of the languid and it lifts up to throw the wind sheet too far out. This is very true of high tin strings of small scale and thin languid flues. A very gentle lowering of the languid is all that is needed to return speech. VERY GENTLE means sometimes only a few thousandths of an inch. Knowing how to do it *evenly* all the way across the languid is something else you need to master.
Lastly, pipes are voiced "fast" and "slow" which to a voicer are the terms to denote where the wind sheet is. Fast is moving the sheet in too much with the unwanted consequence of making the pipe easy to over blow to a partial; and, slow means the wind sheet is too far out so that oscillation comes on slowly. This is why a voicer with a pipe on a voicing jack will tap the key to make the pipe repeat and listen to for the starting transients to be correct. Burbling, slowness, unwanted chiff, etc., are all symptoms telling a voicer where the languid is versus where it should be.
At one time most of my business was in pipe repairs and re-voicing ranks screwed over by amateur weekend jack knife voicers. After seeing many fine ranks ruined I sold my voicing table and now just melt screwed over pipes down for regulator weights. The moral of the story is that if you don't know what you are doing, don't do anything at all. The last straw for me was a three rank set of Ernest M. Skinner strings someone had decided on which to increase the cutup for playing on a higher pressure. BER - beyond economic repair......
Much of this was oversimplified because this is my busy season running around doing the tuning for the services... hope it helps somewhat...
Best of Season's Greetings to All...