coldquilter

Ok, I hate to keep asking questions and be the local pain...... I asked the question about keeping a quilt square about a week or so ago and someone gave me the good idea to use some cheap painters tape on the rollers to keep an eye on what the quilt did as I rolled it. Great advice, sorry I don't remember names well so don't remember who said it. Anyway, I put another quilt on yesterday and was working on it tonight and put the tape in place and what is happening is that the farther down the quilt I go, the middle is shrinking in. Does anyone have any helpful ideas on this? I would imagine it was my problem all along but I didn't realize it without the tape in place. I have not been doing this very long so it is a learning stage right now and I don't really have anyone else to ask so I really, really appreciate all your help. I always try to answer other questions that I can from other beginners hoping to make some points for all my dumb questions. One thing that I thought of, not sure if it makes a difference on the long arm if the selvage is running lengthwise or across the top and bottom. This is something that I have done different on a few quilts and some have mysteriously worked out better than others and so wondering if it might have something to do with it. All good fabric that I am using. Thanks so much in advance. Can't wait for advice.