У меня было такое, когда я пытался соединить U параметр с V параметром.
Проболема изчезла, когда я сделал Reverse Surface Direction->Swap, для одной из поверхностей.
One or all of the steps may improve the probability of succesuful result:
1. Select the Isoparms on both surfaces to indicate the edge of future attachment.
2. Delete History for all surfaces involved to operation. Check HyperGraph to make sure it is "clean".
3. Make sure the UV direction is the same on both surfaces.
4. Choose the "Keep Originals" option in the Attach Surface dialog box. Even if the resulted surface is not visible in the viewports some new object/node can be found in Outliner. Select that guy and open Attribute Editor. Find Attach Surface node/tab and try to change the options/values. Something like changing type of attachement from "Connect" to "Blend".
That is the first came to mind about the attachements.