Minecraft wiki entry for Redstone_tips, associated text: What part of the wiring actually is not behaving as expected?. Unexpected output behavior is usually only a symptom, where the actual problem resides somewhere in the wiring.
Are signals out of sync due to timing issues?
Are parts of the circuit activating when they shouldn't be? Maybe accidentally "crossed wires" are allowing a signal from one part of the circuit to activate another part of the circuit, or a repeater's output is being cycled back into its input.
Has the wiring been damaged by pistons pushing the wiring around?
Trying to draw power from a weakly-powered block? Maybe a redstone repeater is needed to either strongly-power the block or to pull power out of it.
Trying to transmit power through a non-opaque block? Replace it with an opaque block, or go around it.
Was a short-circuit created and a redstone torch that should be powered is now burned out? Fix the short-circuit and update the torch to get things going again.
Are pistons, dispensers, or droppers being indirectly powered when they shouldn't be?
Is the circuit based on a tutorial from an older version of Minecraft which no longer works in the current version?