I had similar problem. May be same as yours. Our original flood lights ar e ~ 10 years old. Bulbs work fine. I purchased color flood lights for hol idays. Plugged them in and none of them worked. tested them in indoor lam p socket and they worked fine. On close inspection the original bulbs had an extra length of thread compared with the news ones. I suspected the new bulbs were not making contact in socket. An electrician clued me in on pu rchasing socket extensions. I purchased a Leviton Standard porcelain exten der from Orchard Supply ($4.95) also here on Amazon:
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Worked like a charm!! Hopefully this solves your problem.
Cheers!