It seems that if you have a storage bin with sweep on that is holding seeds, but some are already on the floor beneath the storage bin, that the pips refuse to plant the seeds under the bin (but will plant other seeds nearby).
They walk up to the seeds, take one in their mouth (chubby cheeks animation) but instead of moving to the planting location simply freeze for a few seconds before dropping the seed. They will repeat this endlessly every 10s or so.
The fix is simply to destroy storage bin. I've seen this occur in two different locations, and in both locations as soon as the offending storage bin was destroyed the pips continued planting again fine.
Steps to Reproduce
This issue has not been confirmed by a developer yet.