How many detonations can I get from a single warp?

You are only able to trigger a biotic explosion with a setup power (setup powers being stasis, warp, singularity, reave, pull, lift grenades, and shockwave(only if you get the lift shockwave at the sixth upgrade)) and then followed up by a finisher power (finisher powers being throw, warp, reave, shockwave, cluster grenades, biotic charge, and nova.) A biotic explosion occurs when a target still affected by a starter biotic is hit by a finisher biotic. what you may be experiencing is that your starter power is still in affect and you just happened to get off multiple finisher powers in that time period and when it didnt happen you probably missed or the game bugged which happens on occasion.