Most filters apply only to RGB images. Check the status of Image: Image > Mode and change it to RGB if needed. A few filters have other requirements. For example Image: Filters > Enhance > NL filter is grayed out if the current layer has an Alpha channel.
If you’re trying to edit an image and your actions seem to have no effect, check the following points:
Is there a floating selection? If so, you need to either delete the floating selection or place it into a layer before doing anything else.
Is the layer you want to change the active layer? The active layer is emphasized in the Layers dialog.
A selection may not be visible either because it is too small or because it is not shown. Try pressing
to toggle selection visibility or 
to cancel any existing selection.Does your current layer have an active layer mask? If so, you’ll see a thumbnail of this layer mask framed in white in the layer line of the Layers dialog. Click the layer thumbnail to make the layer active.
Is your current layer opaque and in normal mode? Check its OPACITY slider and its mode in the Layers dialog. If needed, push the slider to the right or change the mode.
Is there an active Paint Dynamics? For example, Basic Dynamics correlates Opacity with Pressure, and if you’re using the mouse, the pressure is very low pressure. Depending on what you intend to do, either switch to Dynamics Off or use the tablet stylus.
You’re probably changing an invisible layer. An invisible layer can still be active, which is indicated by the emphasis on the layer in the Layers dialog. Any action you take is performed on the active layer. Visibility is indicated by the eye icon on the left of the layer line in the Layers dialog. If the eye is not shown, the layer is invisible. Click the icon to restore layer visibility.
The selection is probably still active. If so, you won’t be able change the channel areas that are outside of the selection. Make it inactive by pressing or selecting Image: Select > None.
The channel is now active, so you have no active layer. In the Layers dialog, select the layer you want to change.
Because the layer is active and the layer mask is not. Click the layer mask thumbnail in the Layers dialog, and ensure it is framed in white, which means it’s active.
The quality of a JPEG is actually not a percentage; it simply happens to have a range from 0 to 100. Saving at a quality level greater than 85 (the default in GIMP) is generally pointless. Digital cameras often save images at a higher level, so saving the image at 85 can decrease its size without a visible loss of quality.
If you choose a deep blue, for example, but the line is drawn in black or white, your image is probably in indexed mode and that shade of blue isn’t in the current indexed palette. Generally, you want to work in RGB mode and change to indexed mode, if necessary, only when the image is finished. The major exception to this rule is when you are preparing an image like a textile design that’s restricted to a very limited color set.