Martins but no nest building

[Woodhook]

I have 6 plastic gourds mounted on an extendable pole. Two years ago I had as many as 11 martins , and last year I could see 8 or 9 every day. For two years, I don’t see any completed nest when I clean the gourds out and no sign of eggs or babies. I have 6 more gourds to add this year hoping to attract more martins. The pole is at least 60 feet from any trees and is about 15 feet above ground. There is a sparrow/ finch bird that likes to build in one of the gourds but I continually remove that nest. Is there anything I need to do or any suggestions? Thanks

[Spiderman]

How high are the gourds?

Ten feet is high enough, but honestly the higher the better.

[SSMartin]

What brand of gourds? There are some small plastic gourds that they generally won’t nest in if there are better alternatives in the area.

[Woodhook]

The gourds are around 15 feet high. I have them mounted horizontally spaced 6 inches apart. The gourds are Royal Wing that I purchased from Tractor Supply.