What is the reason why solder paste plugging is produced in PCBA production?

The common reasons for the plugging of the solder paste are as follows:

1: the stencils has not been polished.

2: the thickness of steel sheet is unreasonable, 0.1MM is better than 0.12MM.

3:PCB positioning PIN is not smooth, so it is recommended to make tin stamping fixture.

4: there is a gap between the stencils and the PCB plate.

5: check whether the scraper is properly installed. Is there any reverse problem?