You oiled the machine, did you put a drop of oil in the bobbin race. That should be done with every bobbin change.
New Needle, Hum-m-m. What size are you using. Remember that you are going through more than two pieces of cotton fabric. A larger needle is required like a 14 or 16. Some are referred to jeans needles. Feel the tip of the needle, is it bent, blunt, is there possibly a small burr on it. Even new needles are damaged in manufacturing.
Now a dumb question, did you put the needle in the right way. A lot of times you can mistakenly put it in backwards.
What size thread are you using. Is it the same as what you are using in the bobbin. If the needle is too small the thread will not ride in the long groove on the needle.
You say you vac the machine, is there possibly a piece of thread still caught in the feed dog area or in the bobbin area and you can't see it.
Did you forget when you re-threaded to include the last thread guide the one just above the needle.
Is the quilting area being stretched too tightly. For instance has the quilt slipped of the edge of the table and the weight of it hanging off the table causing the quilting area to become too tight.
Have you cleaned out the tension discs, yes they collect lint and can cause skipped stitches also.
What kind of fabric are you using, batik, (g-r-r-r) How about the batting is it cotton or poly. Poly heats up with the friction of the needle passing through it and can cause skipped stitches. How about the right and wrong sides of the batting. The rough side should be down or facing the backing.
How about some silicone on the thread.
Are you moving the quilt too fast under the needle.
Fabric with too high a thread count like sheeting or batik can also cause skipped stitches.
These are a few of the things that have happened to me and I've spent many a frustrating day trying to figure out what has gone wrong only to find out I used too small a needle or it was just slightly bent or a bit of thread has gotten caught in the bobbin area or feed dog area and I knew I had cleaned it completely.
Hope this helps.
Kate T. South Mississippi