Some useful tips to try can be found at http://www.embl-hamburg.de/services/protein/production/expression/optimising_exprlevels.html
I've had a recent case where an untagged protein (part of a complex) was not expressed at all but expressed well when tagged at the N-terminal with His6 or MBP. Also try changing from an N-terminal to C-terminal tag (or vice-versa). ho UC Berkeley
