1.SIM card circuit, USIM_VDD=0 reason is that the SIM card could not be detected. The module search SIM card sequence is 1.8V SIM card -> 3.0V SIM card, if 3.0V SIM card is not be detected, than the USIM_VDD output low(0V).
2.If some of your device works flawlessly, some of them never connect or work discontinuously, I suggest that you had better mount the 15K pull-up resistor, this resistor can anti the interference from outside.
3.Check your TVS diodes whether its parastic capacity is over 15pF or not, if it is over 15pF, than consider to replace it with not over 15pF’s.
4.Check your design whether follow the criteria as below:
