Modem sim800c is not working

Dear Sir/Mam

I am working on sim800c modem and out of them mostly they are working well.But I am facing some issue with some modem that is when we connect it to power supply then on net light and status light pin have 0 Volt as well as modem is in waiting modem to start on serial monitor.

Please give me some idea to rectify these issue.

Which module are you using?