Are there any defenses to failure to appear?

In Vancouver, Washington, if you never personally appeared and a summons was simply issued to appear, it may be difficult for the prosecution to prove that you knew - they would have to prove you knew to convict you. However, if you signed a paper agreeing to appear and forgot your court date, it is generally pretty difficult to beat a bail jumping charge because the defendant forgot. As a last resort, a technicality may fly at trial, but the chances of winning are generally slim if you just forgot. If you have a good excuse for missing court, you may have a defense. As you have probably learned, it is very important to carefully keep track of your court dates, once you have been told you have to be in court. Prosecutors often file bail jumping charges, which typically makes it even harder to resolve the case.

If you are a defendant, you are expected to advise the Court (and your lawyer) if you move, and were supposed to be informed of this by the Court at arraignment. Whether an "I forgot to advise everybody, including the Court of my new address" defense will work, depends on a number of factors you do not list, such as the time period from the issusance of the warrant until now, your prior BW record, and whether you are intending to surrender to the Court rather then waiting for them to find you and arrest you on the warrant. Good luck.

DISCLAIMER
I do not practice law in your State. This answer is provided solely for informational purposes only. This answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising.

Moved and did not get summons is among the worst. Why? It's just not credible that with the efficiency of the postal service, that if you had moved and had your mail forwarded you would not have received the summons. I would try "the dog ate the summons" before I trotted out that BS. Medical emergencies work, being in custody in another jurisdiction works, death in the family may work. You are expected to know your legal obligations and observe them. Oh another is I was run over by the bus outside the courthouse.