Layers can be tricky, and it is sometimes possible to run into what seem like incomprehensible difficulties while using them. The following list of problems addresses some common difficulties with layers. In a nutshell, the information in the Layers dialog almost always contains the solutions to these problems.
GIMP operations are applied to the active layer. A common problem with layers is that the active layer is not visible. It is possible that the active layer is blocked by layers above it or that the visibility of the active layer is not turned on. To solve the problem, make sure that the Eye icon of the active layer is on, and turn off the Eye icon of the layers above it.
If you are trying to draw or paint on a transparent part of a layer, and nothing seems to leave a mark, the problem is that the Keep Trans. button is on in the Layers dialog. Toggle this button off and your problem will...disappear.
Background layers and flattened images (see Section 2.7.2) have no alpha channels and so cannot have a layer mask. This problem is easily solved, though, by applying the Add Alpha Channel function from the Layers menu.
A floating selection disables all the other layers until it is anchored to a new layer or to the last active layer. To anchor to a new layer choose New Layer from the Layers menu or click on the New Layer button in the Layers dialog. To anchor to the previously active layer, choose Anchor Layer from the Layers menu, or type C-h in the image or dialog window, or click on the Anchor button in the Layers dialog.
Not all image formats support layers or transparency (JPEG, is one example). For finished projects that are saved to formats supporting alpha channels, use the Merge Visible Layers function from the Layers menu before saving. If alpha is not supported, use Flatten Image from the Layers menu instead. Also, read the next item.