I agree with all the suggestions given. I haven't seen an answer for SEWWHAT's question.
Do you press your seams to one side? Do you nest them (one side goes one way and the other side goes the other way)? If both seams are going the same way when placed together to sew, the bulky seam could be making your seams moving when going under the presser foot, no matter how much pressure is applied.
Can you post pictures of how you're lining them up? Maybe we can think of other suggestions.
Nan - Indiana