Dargyn Kills Not Counting On Riven

[(PSN)Delmonte_Trepes]

Got a Melee Riven Mod that requires you to kill 4 Dargyn pilots before they hit the ground. Ive killed more than 4 in a single day or session in this way but not a single one has counted. Had this Riven for a couple months now but not once whenever I try to do it that it has counted the kills. The only time that i saw it count the kills is the day I got the riven but it became night before I finished it and they stopped spawning. So I logged off to try it the next day and my kills have never counted since then.