Many physical factors can effect the development of the crystals.
The most common ones are;
Resting Temperature, meaning the temperature at which it does it growth.
Melting temperature, Alum one need a 93C to melt, not quite boiling.
Air Currents, if its resting in a drafty place you may get varied results.
Large crystals show the Octahedra Shape well but don't cover the entire pipecleaner because most of the Alum has been used to grow crystals larger instead of growing more crystals.
Complete coverage, though crystals were a bit small.
This snowflake was complete on a day where a cool breeze was coming through.
This Snowflake has wonderfully sized crystals, that show shape and spakle, yet the crystals are still small enough to give full coverage to the pipecleaner frame. This piece was grown in the most unexpected of places, The Fridge!
If your not happy with the snowflake you can melt it down and try again!