Probably just your bad luck? On average you should be getting a relic mostly every 3, rarely 4 encounters, however, if you received above-average drops in the past, don't be surprised to see lower drop rates as probability will always round out the further you go. It happens with my BG as well, I sometimes get 1 in 5 buildings x2, sometimes 4 in 5 when my set BG rate is 1 in 4