There are some problems with the refreshing of various timers when using a Scourge Warlock.
When you use Soul Scorch mechanic (Tab key) paired with Soul Spark Recovery feat (Every 6 soul sparks spend on Soul Scorch reduces your encounter cooldowns by 1 second), if you use any encounter (like Killing Flames) after using Soul Scorch then the cooldown reduction is cancelled and not applied.
Soul Puppet timer (20 seconds) is not refreshed if you do almost anything before its timer is refreshed after using Hadar's Grasp. Furthermore, there is no good in game indication about the Soul Puppet timer being refreshed so you basically have to stop attacking for a while if you want to be sure the Soul Puppet timer is refreshed appropriately.
Because the Soul Puppet timer is not refreshed, the stacks of Soul Investiture are not refreshed either, so the bonus damage is lost.