Normally this problem occurs when not enough divisions and runs have been simulated as part of the Monte Carlo process. RORB normally fails to extract a hydrograph when it cannot find a representative run from the Monte Carlo simulation. When more simulations are run, there is a higher chance that a simulation that fits the criteria to be exported will occur. The process in how RORB chooses a representative hydrograph is detailed in the release notes.
If this doesn’t solve the problem could you upload your .catg,.par and all the other input files or send it through the enquiries email.