I compared with Samsung galaxy it shows full strength n mine shows only 3 bar. Some of my friends told that signal coverage will be bad in Lumia series while compared to Samsung Sony etc.is it true???
I've got a friend who lives just one street over from me. When I go to her house my signal is always dodgy at best, always 2 bars on my 925. But she always seems to have full bars and LTE available. Let's me question the build of my device.
The only other option is to try a hard reset or use the Nokia Software Recovery Tool but there's no guarantee that either of these two things will solve the problem.
My wife's SIM card went bad and we replaced it but the OP had tried a different SIM so I'm pretty confident it isn't a carrier issue. More likely a OS or firmware but can't be sure.