Choose your answer before opening the explanation. This is a learning exercise, not a competency assessment.
Your pins have bulbous bases and very small caps. The most likely cause is:
- Carbon dioxide too high for the pinning stage
- Casing too thin
- Compost too old
- Spawn of the wrong strain
See the answer and why
Carbon dioxide too high for the pinning stage DMR links onion shapes at pinning to high carbon dioxide.