Vodafone Callya card no good connection?

My Vodafone prepaid SIM is in the second SIM slot, and this SIM doesn't have a good connection. I have to keep opening and closing the slot and then re-login to my phone with the SIM to get a good connection. Is anyone from Vodafone here? Why is this? I've already checked the internet connection at my location; there are no problems. Why does the connection always get bad and then go back to normal after re-logging the SIM?