What are the factors that cause SMD components (e.g. SD card holder, USB socket) not to be tinned? How to control?

SMT components do not appear on the tin or tin bad problem has many reasons

1 PCB problems, may be stored for too long leading to oxidation or seasonal changes in moisture easily lead to tin bad

2 may be the front of the SD card holder of the big foot tin unevenly caused by the bulk of the bad

3 process problems, looking for engineering analysis analysis