Problem with both top and bobbin thread?
 

I was sewing string blocks and didn't notice any problems because of the thread color. However, on the last one, I switched the thread in both to another color and then noticed the bobbin tension was too tight. Loosened it a quarter turn (Janome 770), changed the bobbin thread to another color (same brand, same weight) and then the stitches were the same. I did the stretch test and it broke in the same place, and I did a zig zag on doubled fabric and it was ok, but still showed here and there on the bottom, and in a couple of places, the bobbin thread showed on top. Then I doubled some fabric and did some straight stitches and the top thread shows clearly on the bottom and the bobbin thread shows clearly on top. Not a lot, just a dot where the needle goes through.

Is this normal?

Everything I've read said the two stitches should meet in the fabric and not be visible. When I tripled the fabric, the top stitch looked fine, but on the bottom, I could see the loop of the top thread and the bobbin thread looked flat on the fabric, like it wasn't pulled into it at all.

Wouldn't it be the bobbin thread too tight if I see more of the top thread on the bottom of fabric?

I think some photos would help us see what is happening.....I am a bit confused. Are you using any batting yet or is this problem just in the stitching of the strings together.....one or two layers of fabric doesn't provide much middle for the stitches to met in. what you are seeing may just be the hole and shadow created by the needle. sometimes stitches will look like the bobbin thread is coming up to the top....but once the piece is washed...this disappears....I always like to somewhat match the top and bobbin threads in color and weight....as to your question. yes you will see more of the top thread on the bottom of the fabric if your bobbin thread is too tight....there are a lot of good video's online if you just google or search for thread tension problems.....

I'll take a photo in a few minutes. No batting, just regular piecing for now. I did google it, even googled tension on both, but all the results were for one or the other.

For perspective, this is a sample I made. I put the phone very close for the other two photos. White is top thread and red is bobbin thread. Both same weight and brand.

I ripped out most of the quilting I did last night and redid it a few minutes ago. This is the back, with the red bobbin thread. I have to look close, but I still see the white top thread in most of the needle holes. This is with the walking foot and poly batting about 1/8" thick.

Is this normal?

I don't think the tensions themselves look too bad.....wondering if it is the thread and fabric combination....what type of thread are you using...brand and weight....and the fabric looks rather loose weave but that maybe because of the photo.....what brand is the fabric and batting and content....those things can make a difference too....

I'm pretty sure that when that piece is washed, the threads will 'sink in' and will not be noticeable

I agree with SewingPup. All of those things can make a difference. Also, I have several machines and some of them don't like a different thread in the top and bottom so that could be contributing to your problems.