This can happen for various reasons. A few pics would be cool, if you're able.

Most likely they just stalled. If a large portion of the jar is colonized, you're probably good.

Are you planning on growing straight from the cake, or using the cake to inoculate your final medium? Either way, it's probably best to cut off the uncolonized portion before proceeding to the next step. Be sure to use a sterilized knife. I have done this in the past with no issues, but I also was not growing straight off the cake.

A lot of the jars from kits have a bit of extra vermiculite on the top as a buffer layer to prevent contamination.

Pics would be nice but my guess is that you're good to go.

yeah don't open them till they are FULLY colonized unless they are pf style jars and it's just the VERY top layer not colonized.

Like others said, pics and maybe we can help.

This happens often, if the sides and bottom are covered. Take it out carefully for it to start fruiting.