It's all gone bad

Since getting loads of advice and help off this site when I first bought my brno #2 (and couldn't shoot for toffee), and putting in lots of practice, I've got used to groups which are more-or-less single holes at 50 paces; headshooting bunnies out to 80 yards with ease; and even the occasional 100 yd+ bunny.
Now it's all gone to pot! Literally overnight. One day it was fine, knocking over rabbits as usual, then the next day it was all over the place. Missed every rabbit I shot at. Went home and tried it on a target, which ended up looking like it had been raked with a machine gun not punched with a precision rifle!
At this point I ran out of ammo. At long last managed to restock, and have another go. Gave the barrel a good clean, which improved things a bit - but not enough to trust it on rabbits beyond 50 yards. Tried firing a group without the mod (a parker-hale), which was much tighter, so dismantled the mod and cleaned that too - it was really crusted up. A lot better after that, but still big groups - as much as 1.5" at 50 yards, which is no good.
I did notice, when cleaning the mod, that one of the baffles in the middle of the stack has been clipped slightly. This hadn't happened last time I cleaned the mod, and looked fairly recent.
So, how can one baffle in the middle be clipped and all the others be ok?
What could have caused a bullet to clip it?
Is this slight damage sufficient to wreck my groups?
Or is there some other underlying cause that's resulted in both the poor grouping and the damage to the mod?
Any suggestions welcome...

Since getting loads of advice and help off this site when I first bought my brno #2 (and couldn't shoot for toffee), and putting in lots of practice, I've got used to groups which are more-or-less single holes at 50 paces; headshooting bunnies out to 80 yards with ease; and even the occasional 100 yd+ bunny.
Now it's all gone to pot! Literally overnight. One day it was fine, knocking over rabbits as usual, then the next day it was all over the place. Missed every rabbit I shot at. Went home and tried it on a target, which ended up looking like it had been raked with a machine gun not punched with a precision rifle!
At this point I ran out of ammo. At long last managed to restock, and have another go. Gave the barrel a good clean, which improved things a bit - but not enough to trust it on rabbits beyond 50 yards. Tried firing a group without the mod (a parker-hale), which was much tighter, so dismantled the mod and cleaned that too - it was really crusted up. A lot better after that, but still big groups - as much as 1.5" at 50 yards, which is no good.
I did notice, when cleaning the mod, that one of the baffles in the middle of the stack has been clipped slightly. This hadn't happened last time I cleaned the mod, and looked fairly recent.
So, how can one baffle in the middle be clipped and all the others be ok?
What could have caused a bullet to clip it?
Is this slight damage sufficient to wreck my groups?
Or is there some other underlying cause that's resulted in both the poor grouping and the damage to the mod?
Any suggestions welcome...