No Self-Proving Will
For a will to be valid in Washington, it must be in writing and it must be signed in front of two witnesses who also sign the document in front of the person executing the will. RCW 11.12.020. You’d think that would be enough to have the will admitted to probate at death, but it's not. The two witnesses are required to come to court and testify regarding the will signing ceremony. Of course that’s not very practical. Therefore, if the will was prepared by a lawyer, he or she should have included a separate page or provision at the end of the will. In that “attestation” page, the two witnesses should have sworn under oath that they were present when the will was signed, the testator acknowledged the document to be his or her will, the testator requested that they sign as witnesses, they signed in front of the testator, and that the testator appeared competent. With that, you have a “self-proving will.” The family does not need to track down the witnesses to admit the will to probate and have the testator's wishes followed. The attestation provision/page is enough.
If you don’t have a self-proving will, you need to find the witnesses and have them come to court with you to testify about the will signing. Or, you can have them sign an affidavit or declaration swearing that (1) they were present at the signing of the will, (2) the testator asked them to be witnesses, (3) they signed as witnesses in front of the testator, and (4) the testator appeared competent when he or she signed the will, and (5) the testator understood the document to be his or her Last Will and Testament. A sample Declaration can be found here (PDF) (Word).