mycelium only colonized the top part of the wheat jars

Hi,  i been spreading wheat Spawn into new wheat . And i realized that  the mycelium only colonized the top part of the wheat jars. It started growing ok, but then stopped and it didnt colonized The bottom part of The jar. What's the reason for this and what should I do? It's been about 25 days and bottom still hasnt colonized ? Should i put them in The refrigerator?

Thanks

R.