It will not mix
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This is the most reported problem in the category and the one least related to how good the formula is. It is also mostly solvable.
The complaint that costs you reorders
Lumps in the shaker, grit at the bottom, powder floating on top. It affects every use, it is visible, and it is what people write about in reviews.
A product with excellent ingredients that mixes badly loses to a worse product that mixes well, because mixing is experienced twice a day and ingredient quality is experienced not at all.
What is physically happening
When fine powder meets water, the outer particles of a clump wet first and form a gel skin that seals the rest away. The inside stays dry indefinitely.
Finer is worse, which is counterintuitive. Very fine particles have more surface area and more cohesion, so they arrive at the water already agglomerated.
And a powder that has taken up moisture in the tub has begun to agglomerate before the customer opens it, so mixability degrades over the life of the tub.
Which ingredients are worst
Anything that gels: certain fibers, gums and thickeners will form a skin almost instantly. Anything hydrophobic, which resists wetting at all. And unmodified proteins, which is why instantized versions exist.
Greens and fiber blends are the hardest category, because they combine both behaviors.
What to change
Buy instantized where it is available. Agglomerating particles into porous granules that water can penetrate is a process done at the ingredient supplier, and it is the single biggest improvement available.
Lecithin or a similar wetting agent at a small percentage.
Control moisture in the tub, since agglomeration starts before the shaker.
And fix the instruction: liquid first, then powder, with a stated volume. Most customers do the opposite, and a product that mixes well at 350 ml and badly at 200 is an instruction problem rather than a formula problem.