Mongodb 2.0.5 sharding no longer balanced

Mongodb 2.0.5 sharding no longer balanced

I started to see a problem a few days ago where the data going into mongo
no longer spreads across the cluster. Everything except a very small
amount of data is going to the database's primary shard even though the
shard key hasn't changed. The shard keys are MD5 sums of another document
field, similar to the hashed shard keys in v2.4 of mongo, so there should
be more than enough variability to spray documents across the three
shards. We're running 2.0.5 currently.
I can't find anything in the config database that would indicate why the
documents are only going to the primary. We create a new collection each
day and write roughly 40 million documents into it. I've verified they are
sharded, and the balancer is slowly moving chunks from the primary but not
fast enough to keep up with the write rate.
Each server that writes into mongo has its own mongos instance, and there
are a few mongos instances for processes that read data. The total number
we have running is a bit over 25. Could the number of mongos instances
cause this problem? It seems like I need to manually define the ranges for
sharding, but that seems problematic to me. I'd like to keep auto-sharding
in place. Where in the config database does this information reside? Is it
possible to see what the ranges are that the mongos instances use for
auto-sharding?