Isp Decimator, working as intended? Options

[Profanity]

I bought myself a ISP Decimator since my new H&K Switchblade 100w head makes this sound that makes me think of trains "choo"ing when im not playing. people told me to get this, and since i just spent that mutch money on a great amp, i was gonna do the same on a great noise gate. but it feels like its not working correctly..

so here is what i did:
connected head to cab
guitar to decimator to head
turned on everything and sett the amp to where it should be when the band have rehearsals
turned on the decimator and put it to where it stopped the "choo"ing sound while just muting the strings with guitar volume on full

now, while i was playing, i noticed that the "choo"ing continued like half a sec after muting the strings, wich irritated me abit, but at least it did something. but then when i did a powercord in the middle of a song we are going to play, and the length of the tone was drastically shortened.

i bought it to get rid of 1 problem, but now i fixed 1 halfway, and got a new problem as well. is this what i payed for or is something wrong? something i could do to make it work better?

as said, im using a H&K Switchblade 100w head and emg's on my guitar. please help, not long before our first gig and i want at least the sound to be as good as possible.

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