You possibly had a contamination, probably chloride was there. Wash well the glass in what you make the mix with distilled water, perhaps it had Cl salts from drying marks, etc, or water was not distilled.
If you have any chloride or other halide in water, silver nitrate reacts to form an insoluble precipitate of silver chloride or the respective halide.
So I'd try to get water that's well distilled for sure to make the mix and also I'd wash the glass with it before mixing.
Silver nitrate still has a lot of solubility by 10ºC :
https://saltlakemetals.com/solubility_of_silver_nitrate/ , so I guess that's not temperature, more likely it's a contamination, IMO.