Stuff I have encountered when working with 555 timers and flashers:
- Check the timing capacitor, it may be too low value
- Check the timing resistor, too big and not enough current is getting to the cap, even if timing equation is correct
- Check voltage connections, both + and ground
- sometimes you need to add a small value cap to the pin 5 of the timer to gnd (can't remember if its actually pin 5, but it should be in the datasheet)
- if that don't work, try changing the timer IC
- if that don't work, try assembling again in a different board
- if that don't work, check the wires, sometimes they seem fine but are broken inside and don't conduct.
- if that don't work, start over with all new components and run thru the checklist again if it still don't work, maybe the design is wrong or you are wiring something differently than intended.
Good luck!