This problem just cropped up on a previously working install - Windows 7, LR 5.6
The large display of the image in the develop module does not update for any of the actions selected in the develop module. Pick a tool, make an adjustment - you cannot see the results of your adjustment on the large image. The image in the filmstrip does display the results of the adjustment. Selecting the crop tool causes the image to go grey.
Using the simple tools in the library module - they all work and show the proper results.
I have uninstalled LR and re-installed.
I have purged previews.
I have deleted "Lightroom 5 Preferences.agprefs".
Further info - Full Screen mode in the Develop Module - it's just black, no image. FS mode in the Library module works correctly.
I have tried loading the latest video drivers for my video card, rebooted multiple times, etc. Frustrated.
I should also note that this problem started right in the middle of an editing session, everything was fine, and then suddenly, the image stopped updating.
Re-Calibrated my monitor - problem solved!
Not sure why re-calibration would solve it, but everything appears to be working now.