SIM900 Quad Band GSM GPRS Shield NETWORK PROBLEM

Hello!

i am new in the arduino world.
Ι am using SIM900 Quad Band GSM GPRS Shield for phone calls. It was working perfectly well, but suddenly it stops the connections with network.
The red led flashing every second instead every 3 seconds until now
I tried multiple sim cards which i also tried with a mobile phone and all woks fine. The nearest antenna is around 100 metre. But general i had no connection problem till now.

Plus that sometime also after a few blinking the leds are stopt exept PWRKEY reset botton! But thant hapeen now and then!

Any idea whach going on?

1 i am using arduino uno
2. i also supply the shield with 9v battery extra from power juck

thaks for any help!

If nothing's changed - that's an important if - it may be your cellular company.

thankyou for the reply.
Well ia m using two diferend sim from two diffrent companys