Moved mSD card from raspberry pi B to pi 2 : stats are down

You do need a better antenna or antenna placement - it looks like you have a maximum range of just over 50nm - it shouldn’t be difficult to get 3 or 4 times that wit even a simple antenna placed outside.

See ads-b-flight-tracking-f21/three-easy-diy-antennas-for-beginners-t20177.html

as for stats being down, it could be due to weather or relocation of antenna.