I am using Photoshop CS5 on a desktop computer running Windows Vista with 4GB RAM. Last night I was working on a file. D3 NEF opened from LR3 into CS5 and created some adjustment layers and other empty layers for healing and cloning. After quite a bit of work, the Clone Stamp Tool and the Spot Healing Brush Tool stopped working. I was unable to get them to work until close the file and reopen it. Does anyone know what's the reason behind this problem and how to fix it?

Thanks..