Flavor King Pluot and Burgundy Plums never ripen

So I have multiple plum varieties and a Pluerry and another Pluot that all make fruit all over the yard, but these two have issues. They always set fruit, it all looks good, but they stay super firm and never seem to soften up like my other varieties and they just never seem to ripen to the point I can eat them. Sometimes it’s so late in the season I try to eat one and definitely not sweet. I wonder what is going wrong…

I have Splash Pluots, Santa Rosa Plums, a Pluerry variety and few other plums (dont know the names) and they are all doing well.