Don't use void shards unless it's a 2x void weekend? Don't use any shards unless it's a 2x weekend for that type?
Void shards are identical to ancient, with the sole difference being the pool of champions to summon from is smaller and, generally, superior. You are no more likely to get an epic or legendary from a void shard than you are from an ancient one.
On a normal day, void shards have a 0.5% chance for a legendary and an 8% chance for an epic. The probability of pulling a rare is 91.5%. Therefore the probability that you get neither an epic nor a legendary from 20 summons is (0.915)^20 = 17%. Just chalk it up to bad luck and save your shards for the 2x weekends.