Illustra locking an Index ?

We have some concurrancy problems in Illustra that we think we have tracked down to a table/index level lock on an index. The confusing part is that this is happening when the base table is being updated but the column(s) being updated are not in the key OR the index. We are therefore confused as to why we're seeing table level write locks on the index. Can anyone shed any light on under what circumstances Illustra 3.3 would take out a table level lock on an index whose columns weren't being updated ? TIA Mark ( To email remove "nospam" from address. )