In order to avoid probate, your trust must be funded with assets, generally your real property. This is generally accomplished by transferring the real property into the trust by way of a grant deed or a quit claim deed recorded in the county that the real property is located in.
If the settlor dies without funding the trust assets, the law provides an escape clause by way of a Heggstad petition, which works as long as the property is listed in a schedule, such as a "Schedule A" to the trust.
What happens if the property is not listed in "Schedule A" ?
All hope is not lost, As long as the written declaration of trust contains language similar to "the settlor or trustor assigns all of grantor's right, title and interest in all of his real and personal property..owned by the Grantor, wherever situated", then a properly drafted Ukkestad petition will avoid probate as long as the trustee can prove by extrinsic evidence that the Grantor owned the property.
The best practice is to review your trust documents now, and make any amendments necessary to avoid having to file any petition.