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.
Larger crystals show the Octahedral 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.
These teeny tiny crystals formed from a solution where too much Alum was added at the wrong temperature. When these crystals solidifided the whole solution turned to "sludge"
Complete coverage, though crystals were a bit small. This snowflake was complete on a day where a cool breeze was coming tT
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!
You will loose Alum during this from your growth solution as well as any undissolve grains from the melting process. Reheat your growth solution (do not add water) and add your snowflake, Stir constantly the bigger the crystals were the longer you will need to stir. Once the crystals are fully dissolved, you can place the snowflake suspended over your growth container. Remember to wait for the solution to cool before adding to the growth container.