Getting a Layer to Work
Occasionally, I will run into a problem where the layer I am working with will not accept the changes I am making. Is this a bug in the program or is something else going wrong?
With all the versatility that Photoshop offers, sometimes there are some unexpected resultsalthough the true surprises and bugs are few and far between. In this case there are several ways Photoshop might be keeping you from applying what you'd like, and all are valid reasons (not bugs).
Hopefully, most of these problems can be taken care of by Photoshop error messages. These are not crash messages, but contextually sensitive, helpful notes that let you know what might be going wrong. For example, if you try applying a History brush to an image's eighth layer and the Snapshot you are painting from had seven layers, you will not be able to apply the History Brush. The Can't Apply icon will appear instead. If you attempt to apply anyway, you will get the message "Could not use the History Brush because the History State does not contain a corresponding layer." In other words, if you see the pointer icon turn into the Can't Apply icon, try applying it anyway. The result might be a helpful message that tells you why you can't apply the tool.
Reasons why difficulty in applying tools might be occurring might stem from opacity or mode selections for the tools you are using to Layer Opacities, pixel and transparency locks, inappropriate application of color modes, sampling mistakes (the Stamping tool), and such. Really, you probably need to check only a few things: Layer Opacity, Tool Opacity, Layer Mode, Tool Mode, Image Mode, Layer Masking, Layer Clipping Paths, Clipping Groups, Selection, Pixel Lock, and Transparency Lock. You will note this can be summarized as Opacities, Modes, Masking, Selection, and Locks.
OpacityAny setting for opacity that is too low might keep you from seeing the result. This is especially true if there are multiple opacity settings at work. For example, it is possible to apply a 10% opaque tool to a 10% opaque Layer and end up with a 1% application of the original color. If that color is less than 50% gray in intensity, it might not show up at all.
ModesThere are two types of Mode to be concerned with, Image Mode and application Mode. Image Modes are the color spaces you are working in, and application Modes apply to the Mode of tool and Layer application. Image Modes will need to be compatibleand in most cases this means the same. If you are attempting to stamp CMYK information into an RGB image, you probably will have a mismatch and experience some problems. On the application Mode side, if you are using a Lighten mode and are attempting to apply a black to an image, nothing will happen. Selecting an appropriate Mode (such as Darken) will help get the results you need.
MaskingIf you use Masking in the form of Layer Masks, Layer Clipping Paths, or Clipping Groups, you might actually make valid application of tools that you simply can't see because it is being masked. Be sure you are not painting to a layer that has either a Clipping Path or Mask applied, and be sure it is not in a Clipping Group.
SelectionSelections can sometimes hide in an image: you might be too close to see the selection which is loaded over another part of the image, selections might not encompass any pixels at more than 50%, or you might have hidden the selection and forgotten. In any case, you can shut off the selection by pressing (CmdD) [Ctrl+D]. After the Selection is disabled, you will be able to apply the tool as desired.
LocksThis is probably the lowest possibility, as you have to set the Locks fairly consciously (whereas tool settings can carry over from session to session and are easier to forget). However, if you turn on the Pixel Lock, you will not be able to change existing Pixels in a layer. Be sure the layer you are working on has the Pixel Lock off (unchecked).
With all this checked, there are still other valid things that can go wrong, such as having a Channel active and painting to that instead of the layer you think you are working on. If you are really in a quandary and have checked everything, open a brand-new image and try doing what you want to do there. If it works, rebuild the image a layer at a time to find the problem. If it doesn't work, you might want to re-evaluate what you are trying to do.