Anyone had experience of this....

Trying create an HDR of 3 RAW files.
Import, then go to tone mapping. Get the preview set up to diaplay as I want it then process.
However, after process the image is WAY darker that it appreared in preview. So much so that it's unusable. It's also difficult to predict which setting/images will be affected this way - other times/settings seem to bring them out very similar to the preview.

Any thoughts? Anyone had the prob and found a way round?