Decreasing lipo life?

[Syphonics]

Currently i am using align 12C 1800mah lipo. I did a record and it got a flight of 20 times til date. Flight time ranging from 6 mins to 10 mins max. In the past, after 10 mins the heli then loses it power. But now the battery seems weaker after 5 mins of flight time. Am i discharing too much power? Spend so much on the battery but it don't really perform that well.
I am running on 15t 2800kv.

[Syphonics]

btw the voltage after each flight is at least 10v or more.