let me take a stab at what your experiencing here my guess is the DEC rewards pools have been diminished in their overall supply. DEC is given to players winning ranked matches from a rewards pool this rewards pool is shared amongst all players & when DEC supplies deplete the reward per each player is decreased overall.

The DEC rewards pools are refilled at the beginning of each season when the DEC reward pools are full you earn the most possible DEC per match. Given the popularity of Splinterlands DEC reward pools decrease significantly faster then in the past.

This is my guess of what you are experiencing, keep in mind I'm not Splinterlands Staff just an avid player / fan.