What should be done if the reverse sewing of the sewing machine is not smooth?

We usually encounter problems with the operator's reaction: "After using the new type of direct drive sewing machine for a period of time, the machine's reverse sewing is not normal. When the manual reverse sewing switch is pressed, the reverse sewing electromagnet is attracted, but it is quickly released, so sometimes only one stitch can be reverse sewn, which cannot guarantee the firmness of the sewing at the beginning and end."
At first, the author thought that the diode of a protective electromagnet in the circuit board of the new sewing machine was broken down in response to this problem. After inspection, it was found that by manually reverse sewing, the reverse sewing electromagnet of the new sewing disc machine can be engaged and slowly released with just one engagement. When the electromagnet is released, there is still a "buzzing" sound inside the electromagnet, which proves that it was energized at that time.
Using a multimeter to measure the voltage on the two wires outputted by the electromagnet, it is suspected that the transistor driving the electromagnet is not functioning stably? Is the multimeter displaying only virtual voltage? I removed the circuit board, carefully measured and compared the transistor resistance values of each driving electromagnet, and did not find any abnormalities.
I thought it must be a problem with the circuit board, but after discovering the problem, I was able to directly replace the control box and the manufacturer, but now it cannot be determined. In order to confirm if there was a problem with the control box, we replaced it with another working control box and tried it out. However, the same problem still appeared. We measured the resistance of the electromagnet and compared it with a normal electromagnet, but no abnormalities were found. We even removed the connectors and dealt with them. The electromagnet was released after being attracted for one or two seconds. What exactly is the problem.
When adjusting, we accidentally increased the needle size and pressed the reverse sewing switch, only to find that the electromagnet was firmly attracted; And when the needle size is reduced again, the reverse sewing electromagnet won't be able to hold it firmly. Opening the machine head for inspection, it was found that the principle is very simple, that is, the larger the needle size, the deeper the electromagnetic iron core penetrates into the electromagnetic, so the stronger the suction. The smaller the needle size, the shallower the electromagnet core penetrates into the electromagnet during reverse sewing. When starting to press the reverse sewing button, it may activate a high current that can be pulled in, and then it may maintain a low current. Therefore, the electromagnet will start to release after being pulled in for one or two seconds.
In this way, finding the real cause makes it much easier to deal with the problem. First, find a large screwdriver, loosen the two large screws that fix the electromagnet, and then move the electromagnet slightly to the right. This will completely solve the above problem. Special attention should be paid here that if the electromagnet moves too much, it may cause inconsistent needle codes for reverse and forward stitching.