Signal strength can vary depending on weather conditions, your distance from a mast, and the location of any given building you're in, particularly if it's in a valley or behind a tree. A building's construction and any temporary structures, like cranes, can also affect the strength of the coverage you get.
You can download Opensignalmaps to see the performance of your local mast and the optimum location for best signal.
If you're still having signal problems, these tips might help you:
Try sitting near a window.
Try using your phone in a different room as some rooms get a stronger signal than others, depending on the thickness of the walls.
Try switching your phone off and back on again.
Make sure you have enough battery life as this can affect signal strength.