Can a bacterium express a virulence gene and not produce toxins and what may be the cause of this?
To expand a bit on @Mohamed's answer, there are several mechanisms by which virulence gene products can be up- or down-regulated without changes to mRNA expression.
Post-translational modifications
Adenylate cyclase-hemolysin (AC-Hly) is a toxin produced by the Bordetella pertussis, the causal agent of whooping cough.  There are very few amino acid differences between the AC-Hly of B. pertussis and closely related Bordetella species.  Palmitoylations of lysine residues on the C-terminus of the B. pertussis AC-Hly are necessary for its specific cytotoxic activity.  [Bouchez 2017]
RNA secondary structures
After transcription, mRNAs can form secondary structures by base pairing, preventing ribosome binding and translation.  For some pathogens, virulence gene mRNAs forms these structures, which melt at human body temperature, allowing for fast production of toxins without the need to increase virulence gene transcription. [Narberhaus 2010]