Are your woven 'edges' drawing in? I can help!
Maintaining good, consistent weft tension with the use of bubbling is one of the most important aspects of tapestry weaving, lest your warps become too close together causing your selvedges (your weaving's edges) to draw in. One of the most common issue for a beginner tapestry weaver is encountering this issue, and the scariest scenario is when your warps get so close together that you can no longer create weft-faced cloth while weaving. This means your selvages are narrowing AND your warps are showing through your weft (a nightmare!) The technical name for these bits of warp showing through is ‘lice’. But, if I’m honest I rarely use that term because it grosses me out.
So, let's talk technique. When weaving tapestry you should always create bubbles with your weft once it is inserted between the warps. If you do not bubble your weft, your warps will become closer and closer together, getting worse with each offense, narrowing your selvedges more each time. Conversely, if you bubble too much your warps will become further apart.
Finding that only some warps are getting too close together, creating a mound?
Especially in larger weavings, weft tension issues are usually isolated to one or more small area along the plane of your weaving, When your warps get too close together in only certain areas, this causes your weaving plane to no longer being straight because the area where your warps are too close together build up faster than the surrounding areas, and eventually appear as a mound. It will also result in your tapestry's selvedges narrow. This is because no matter where your tapestry is getting smaller in width (on the inside or on the selvedges), your weaving as whole will narrow.
To troubleshoot these areas do the following:
Make higher bubbles in the problem area. Making higher bubbles in only the problem area of weaving allows more yarn to push the warps apart. It usually takes a few picks of this kind of weaving (as per illustration below) before the problem is solved.
The key here is to make sure you don’t let the problem get too bad before you begin to troubleshoot it. So, as you weave keep a close eye on the spacing between your warps. If you are at the point where your warp is showing through and you have a mound to contend with where the warps are getting closer together, just keep weaving your bigger bubbles in that area, stuffing more weft between those warps in each pick, until the issue is resolved. It will take some time.