As Holice first suggested:

Make sure your feed dogs are lowered/dropped or covered.

On my machine (a Pfaff 1471 - late 1980s machine), the instruction manual says for darning to lower the presser foot only 'half-way' for darning/FMQ. There is a 'stop' that I seldom use, but it is there - perhaps your machine also has the same feature?

Do you still have the operating instructions that came with your machine? There may be a section called "darning" that might give you suggestions on how to set up the machine.

Ok I am going to jump in here with another idea. Read this thread here on the board. http://www.quiltingboard.com/main-f1...ng-t35165.html
Could it be tufts of batting showing through on the back of your quilt and not the tension. Also my sewing machine man years ago showed me the drop test with the bobbin but he showed not to have it drop over a quarter inch at a time. I hope this helps you some.

- I agree with Joset, I am also new at quilting I started January 2012, I have a Janome 1600P and I love it, I have it on my 10' foot quilting frame and it does an awesome job and is super fast and easy to figure out since I am no awesome person at figuring out how to run sewing machines. I too have a mega old sewing machine that I do ALL my piecing on which is an old old ALCO that my mom gave me...I am 27 yrs old and have fallen in love with quilting...I use my Janome for doing all my quilting and fooling around with new designs after I have finished my tops... sometimes in the bobbin case there gets lint in there that can cause thread issues, I use a makup brush to clean it out a bit and seems to help some...tension is always tricky to get perfect and sometimes it doesnt get perfect but after one wash the threads pull eachother to the center and it looks just fine ...best of luck and have fun!

In my humble opinion all you need to do is lots of practice. At times I have to do unsewing also. I have lots of experience but, sometimes it would be impossible to tell it.
Welcome from Missouri.

This is my machine too, and I've been having fun doing FMQ with it. I did have that same thread problem, intermittently, and in addition to what the other gals are saying, check to see that you are not inadvertantly lifting the pressure foot with your knee as you are sewing. For some reason I was doing that, and it really messes up the thread tension. As soon as I released my knee to a relaxed position not on the knee lever, my tension was restored. Happy sewing!

Thanks everyone...all suggestions added to my little note book. Have just finished sewing some Christmas print dresses for my granddaughter and the machine is sewing "a treat" with lovely stitches and tension. So all is back to normal and the Janome it would seem, has forgiven me! Will start piecing my next quilt top in the New Year when my fabric arrives (man, I love Etsy!) and then it is "back on the learning curve" horse! In the meantime I am going to buy a really cheap quilt from KMart and put all your suggestions into practice so when I am ready to start on my quilt I will be all "practised and sorted". I think I am going to have to re-think my sewing table set-up as well and perhaps get my OH to "rig up" a nice big extension for me. There's a nice little holiday project for him!!!! Thankyou all so much....what lovely support this site gives...and Merry Christmas to everyone too!

Point taken Dogwoodquilter........yeah, my bobbin case doesn't really drop a "whole inch or so"...just had another look and I may have got my "drop measurement/guess" slightly wrong....never was too good at judging distances!!!