Xp farm build

Hi
So I built this xp farm sometimes it works some times it doesn’t

I hit the allay with a potion of infestation, lite the fire then a ton of silverfish spawn in often lagging out the game massively.

Sometimes the allay don’t take any damage at all and it doesn’t work, sometimes only one takes damage.

I’ve messed around with the hight of the block that locks them in but can’t seem to figure it out.

It’s often hard to put the fire out also and I accidentally kill allay.

Any advice on how to fix it or make it better would be appreciated.

Thanks in advance