Make sure the entire button is contained in the portal row, and not above, below or hanging off of the portal row, in layout mode. I suspect the button isn't entirely in the portal row. Drag the button completely out of the portal, and then back into the portal row. Also you may have to make the button a little smaller.
A good check is to drag the portal a few pixels while in layout mode. The button should move when the portal moves. (Unless you are using FileMaker 11 or older...)
If that's not the case, you'll need to share your relationships and the precise details of you Go To Related Records script step.
I am having the same problem. Mine is with a script trigger from a cell in the portal row Again I can't get it to go to a related record, it goes to the first record in the related group. It is not a problem of having the cell outside the portal line. I can get it to work by "setting tab order" and telling the field which is triggering the "Go to related record" script to tab to another cell on the portal line but then I can't get it to reference the cell which has just had data entered. In short I do not want to have to use the tab order method, I want the focus to stay on the cell into which I have just input data.