Not every peptide reconstitutes the same way. Some dissolve readily in plain water; others need a more specific pH environment to go fully into solution. The difference traces back to the peptide's own sequence, not to anything wrong with the preparation technique.
Where solubility behaviour comes from
A peptide's solubility is largely governed by its amino acid composition — specifically the balance of charged, polar, and hydrophobic residues along the sequence. A sequence with a higher proportion of hydrophobic residues will generally be harder to dissolve in plain water than one dominated by charged or polar residues.
Every peptide also has an isoelectric point (pI) — the pH at which its net charge is zero. Solubility is typically at its lowest near that point, since the molecule has the least electrostatic repulsion pushing it apart in solution, and improves as the surrounding pH moves further away from it in either direction.
Why a formulation note matters
This is why some products specify a particular diluent or buffer rather than leaving it open — the formulation has already accounted for that sequence's solubility profile. Substituting a different diluent than specified can mean fighting the peptide's own chemistry rather than working with it.
If a vial isn't going fully into solution with the diluent and volume specified, that's a signal worth double-checking against the product's own documentation before assuming the powder itself is at fault.
A practical note on gentle mixing
Vigorous shaking can introduce excess foaming and, for some sequences, mechanical stress that isn't necessary to get a peptide into solution. A gentle swirl or roll between palms is generally enough — patience matters more than force here.