MFF2 SIM soldering Issue

Good morning,
We’ve produced 500 PCB boards embedding 2 MFF2 SIM on each of them. We’re facing this issue: 10% of SIMs after some weeks stop to work properly.
Our tests showed that is a soldering problem, but we developed soldermask and pastemask PCB layers according to your datasheet.
Can you help me ?
Can you give me some guidelines in order to design the best layout for MFF2 SIM (regarding IC footprint, required soldering process etc.)?
Now, we’re programming to produce a new 2000pcs batch but before we need to fix this problem.

I wait your feedback.

Best Regards,

Marco Croci