I am using Arista EDU 400 film and Arista developer 1:1 and following the developing time recommendation of 12.5 minutes. I am careful with cleanliness, times and temperature; everything the same every time. Nonetheless some rolls come out of the tank with low contrast, lacking acutance. Resulting prints are muddy. My developer is relatively fresh (stock mixed within a couple months) and stored in stoppered amber bottles, mixed 1:1 at time of use. Exposure is not the issue, negative density is normal. Film is kept in the fridge, but once loaded sometimes languishes in the camera for months before development (this is what I suspect might be the problem, what do you think?) Any suggestions?