Besides confirming that thread tension is not the problem and that the machine has stitched an upper and lower stitch evenly on the top and bottom of the fabric pieces it may be that the thread itself is an issue.
When the presser foot seems reluctant to advance the stitch, look at how the machine is threaded. Often the spool thread may have slipped off one or more of the machine’s thread guides. Reposition the wayward thread back into the appropriate thread guide or rethread the machine if necessary.
For regular sewing, is the machine thread and bobbin thread from the same thread spool? Using thin serger thread in the top of the machine and regular all-purpose thread in the bobbin may seem a frugal workaround however the slight difference in thickness of the threads can make a difference in how the stitch is created as the machine needle plunges through the fabric under the presser foot and is caught by the bobbin case’s rotation and retreats back up through the fabric once again to complete one stitch. The slight variation in thread thickness over a few stitches can create, at times, a nest of thread under the fabric.
Is the thread used for threading the machine and bobbin of vintage age? Vintage is of course a relative term since there are many factors that could impact the quality of thread besides age. Exposure to light, heat and moisture can impact any thread's ability to resist breakage. Older threads may not have been subject to being mercerized – a mechanized process by which threads are rounded and fuzz is removed. Less fuzz on threads results in less lint, less lint on strands of threads helps the thread to pass through fabric with less friction and avoid bunching of threads into a thread nest. Modern newly processed threads are all mercerized and resist breakage well.
It is interesting to note that some vintage threads have stood the test of time. Cotton, silk or rayon threads, wound evenly on wooden spools, and those carefully stored for decades – some 100 years old, may be the best choice for sewers who are repairing antique clothing and quilts. It is easy to test for breakage by firmly tugging on the thread. If the thread does break, take heart, vintage thread spools make for charming craft projects or just displayed for their historical significance.
Some language idioms – cultural expressions really and those in particular that relate to sewing such as hanging by a thread, a stitch in time saves nine, she had me in stitches, remind us that it is our recognizable expressions of language that are the common threads that connect us one to another.
Sew happy, sew inspired.