Plugged cones send drilled solids back into the mud. A bank with several dead cones is doing a fraction of its job while ‘running' — and unlike a flood, it makes no noise and shows no spray, so it's easy to miss.
Likely causes
Apex too small for the load — chokes and blocks.
Worn / damaged apex changing the discharge.
No trash screen / desander upstream — debris reaches the apex.
Cones run on a mud they're not suited to (very high solids).
LCM or coarse trash in the system lodging at the apex.
Gelled mud setting in idle cones.
Large particles or agglomerated solids blocking the apex.
Check apex size against the load — too small chokes.
Confirm a trash screen / desander is protecting the cones.
Inspect for LCM or debris in the feed.
Check manifold pressure / feed head feeding the bank.
The fix — step by step
Clear plugged cones; fit a trash screen / desander upstream where trash recurs.
Open or replace the apex to suit the load (umbrella, not choke).
Keep LCM and coarse trash out of the cone feed.
Maintain feed head so cones run cleanly (see cone roping).
Flush idle cones to stop gelled mud setting at the apex.
Confirm it's fixed
✓ Verify: Every apex showing a steady umbrella spray after clearing; a trash screen protecting the bank.
Field note. A plugged cone is silent — no flood, no overflow, just nothing coming out the apex. Walk the bank and look at every discharge: the cones that aren't spraying are the ones quietly sending fines back to the mud.