Wp7609 does not enable uim_vcc

The Sierra WP7609 is not enabling the SIM CARD power supply (pin 26 - UIM VCC) properly in our prototype. We noted that right after the WP7609 is turned on, it tries to enable the UIM_VCC voltage (1.8V) for some seconds (verified with oscilloscope) and than it turns this pin off (0 volts). We noted that when the SIM CARD is inserted in the SIM CARD HOLDER, the pin 64 (UIM_DET) receives the logic transition from 0 to 1, and at this moment the WP7609 enables the UIM_VCC voltage for some seconds, than disables it, and at following, enables the SIM RESET signal (pin 29 - UIM_RESET_N), putting this pin in low level state (0 volts).

After this, if we force a transition from logic 1 to 0 at the pin UIM_DET, the UIM_VCC pin is enabled for some seconds, and then is turned off again. We verified that the module is activated, because it answers normally to AT commands sent with minicom.

Could anyone please help me with this issue?