I don't know what machine you have, but with the Janome have your top tension on 2 and NEVER EVER touch the tension screw on your bobbin.That is precision set by the factory. It might also be the bobbin fill you are using. I personally prefer Coats FillMore. I use the white for light coloured embroidery threads and the black for black and very dark embroidery threads.
Hope this helps.
You seem to have your answer already. I hate having to tear out a design as it looks terrible when do but for next time or if I see it start to happen while doing design, I stop machine and put in a new needle and it stops it. I hate when that happens and I don't notice til too late.
Cutiepie has given you the best answer I know. She is very knowledgeable in the world of machine embroidery. I keep those little sharpies on hand all the time, and a clean machine is a happy one. *
arsenio, WELCOME to the CUTE family - cutiepie has given you some really good answers - hope one of them work for you - here is a flower for you.
Do you mean why does the bobbin thread show up on top sometimes? That is usually a tension issue. Check your bobbin case and make sure it is clean, and then rethread your bobbin. You can adjust your top tension to see if that helps.
If you mean how do you "fix" a project where the bobbin thread has pulled to the top (without picking out the whole thing and trying again), assuming that the bobbin thread is white, you can try to cover it using colored permanent markers (like Sharpies) in the color thread it is supposed to be. I have done this with good success, although you do have to be quite careful to not get the marker where you DON'T want it, and some threads soak up the color very easily. Plus, it can be a challenge matching colors if the color you want is not one of the "standard" rainbow.
Good luck solving your problem! I hope that it isn't too bad, and that you quickly find a solution that works for you which doesn't include sending your machine in for repairs. That is annoying. =]
Great answer, Cutiepie. Those sharpies come in so handy at times, like an outline leaving a tiny gap somewhere.