Use 'patchy' to indicate something varies and isn't consistent throughout an area or period. His attendance at school was patchy this semester.
Pro Tip 2/3
Indicate Incompleteness
'Patchy' suggests that something is incomplete or has missing elements. The grass on the lawn grew in a patchy manner.
Pro Tip 3/3
Non-uniformity
Describes cases where elements are distributed unevenly, creating a non-uniform appearance. The patchy coverage of snow on the mountains was unusual for winter.