Pembina/Brooked plum pollination help

So afew years ago I bought a Pembina and Brookred plum , with the expectation they would be able to cross pollinate each other. I hand polinated the two of them with a paint brush. I only got one plum from the Pembina and zero from the Brookred. So I purchased a Nanking cherry and a princess Kay ( prunus Nigra ) plum. I have read some conflicting information about the princess Kay being a hybrid sterile plum , and other information saying it is a wild plum found in massachusetts in 1986. Does anyone know if this plum will be able to pollinate my two hybrid plums?

Welcome to Permies:
I do not have expertise on plum types to say which will cross pollinate. But what I have found to be key to pollination is having the pollinating insects at the right time.  they tend to bloom early while the weather is still cold. here the mason bees are the ones out at that time and they do an excellent job of pollinating the plums

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