Three weeks ago I racked and bottled some perry. Before bottling, I filtered it through a kitchen cloth. I added a teaspoon of sugar per bottle.
With the exact same approach, I got a rather foamy apple cider. The perry however is rather flat, hardly any carbonation.
After filtering, the perry was rather clear. I suspect that for some reason there's not enough yeast in the bottles and I plan to add some. But are there other possible reasons?
Did you sanitize the kitchen cloth you use to filter?
I always do the filtering when passing the wort to the primary because is easier and always with sanitized equipment.
Next time try adding sugar to the whole beer (in the fermenter) instead to each bottle.
It's unlikely that a coarse filter like a dish towel would remove all the yeast from the perry, but it could have removed enough to slow fermentation down. The other possibility is that there's lack of yeast nutrient, which would also cause slow fermentation.
The first thing you should try is waiting longer. Keep the perry in a warm (~700 F.) place. Give it another month to carbonate.
If time and patience don't work, you could try removing the caps/corks and adding a few grains of yeast, and a pinch of yeast nutrient to each bottle. Then wait another month.
On a side note, you should sanitize everything that comes into contact with the perry, even after fermentation is complete. Even though most of the sugar has been metabolized by yeast, there are still bacteria that can affect the perry.
The last time I had trouble with bottle conditioning, I agitated all of the bottles just a little bit. I picked up each bottle, tilted it to the horizontal, gave it a half-turn, then put it back into the box where I keep my bottles.
In a few more days I had good carbonation.
