I have several configurable products with associated products. All have worked well and I am familiar with how to add both.

I have 2 products to add to one of my configurable products.

I added the first one by duplicating an existing item, and making necessary changes all the way through. 

When I need to select it to add to the list of associated items, it's there but highlighted in red and not selectable. 

I could not figure it out so I deleted the simple product and created from scratch. Same thing. 

I can add it other configurable products, but not this one - UNLESS - I uncheck one of the existing ones. There are 5 there now. Is there a limit on how many simples can be associated with a configurable?

I cannot find this question on the exchange, so if you mark as duplicate, please also leave the URL to the answer.

Any thoughts? Magento 1.9x - Thanks for any input.