RMAN 'delete obsolete' command takes ages

I'm running Oracle 10g on RHEL 5.2, backing up with RMAN to Tivoli Storage Manager. When I run 'delete obsolete', it quickly lists what it wants to delete but then just sits there for many tens of minutes, occasionally deleting one.

strace tells me that the rman process is blocked reading from an oracle process, which seems to be doing nothing much. What can I do to find out why this takes so long?

Many thanks for any advice
- Ian (not a DBA, and please don't tell me to hire one!)

-- 
Ian Chard, Senior Unix and Network Gorilla | E: ian.chard_at_sers.ox.ac.uk
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