LeLit PL41-TEM back flushing

I bought this machine 2 years ago and regularly backflush with Cafiza. Havent used the machine in two months and noticed that the backflush pressure is less now. at the 2:15 mark of this video, it performs as it always has on a backflush...you can hear the fluid coming out of the hose in the drip tray pretty much immediately, but after this, it seems to be releasing at about 8 bar. I had noticed that prior to this video it wasnt releasing the pressure at all. The needle would slowly go down and in a few instances there was an explosion from the group head. Thought there may be an issue with the seal so took it out and inspected it and it looks fine. Gave the grouphead area a clean and reassembled and it hasnt done that since.