The game has a a long delay to when it registers you standing in the plume and giving you the buff. Same issue occurs with sacred soil there is about 1-2 sec delay before it shows up as a buff. The solution is to kill the spiny plume 1-2 secs faster then you have been doing.

The best way to do it is to get the plume very low on hp 1 hit away from death usually the off tank can do this so when she jumps 1 hit from any dps can kill it. Its usually groups that leave the plume at high HP when they need to kill it have the issue with people not getting the buff.