The sifter clogs

The jar is fine, the powder pours from the open neck, and nothing comes through the sifter holes. This is the complaint that generates refunds rather than remarks, because the product is unusable rather than merely disappointing.

The aperture is the narrowest point

A sifter is a deliberate restriction. Everywhere else in the jar the powder has room to move; at the holes it has to pass single file through an opening a few millimeters across. Any tendency to bridge shows up there first and worst.

Same mechanism, worse location

It is caking, at the place where caking matters most. Water condenses at particle contact points and forms liquid bridges, and a bridge across a two millimeter hole stops flow completely while the same bridging in the body of the jar is barely noticeable.

Which is why a jar can be scoopable and unsiftable at the same time, and why the customer's description of the problem will not match what you see when you open it.

Grind is half the answer

A wide particle size distribution makes this much worse. Fines pack into the gaps between coarse particles, reduce the space available for flow, and give the bridges more contact points to form across.

A uniform medium grind sifts well. An inconsistent fine grind sifts badly even when perfectly dry. If a line clogs and others do not, look at the mill before you look at the humidity.

Choosing a closure that works

Match the aperture to the grind rather than to the range. Leaf and coarse grinds want larger openings or a spoon aperture; fine powders want more holes rather than bigger ones. A dual-flap closure that offers both sift and spoon solves most of the range in one part.

Then test with your own worst case. Fill a jar with your coarsest leaf and your finest powder, leave them a two weeks, and sift them. The closure that survives that is the one to order in quantity.

References