It looks like the UI for a blueprint “SetScalarParamerterValue” is not working correctly.
It seems to only be displaying a few entries (scalar values) even though there are more available in the collection. I was able to verify that they are there and working by manually creating a variable and wiring it to the input on the node but they just can’t be selected from the menu because it doesn’t roll out far enough. You can even use the arrow keys to select beyond the bottom but it won’t accept an enter key input to actually choose those items.
There does appear to be a bug here and I have alerted our engineers to this issue, for reference UE-12322. But to keep you working there is a work around to access those new parameters created. If you reassign the collection to the Set Scalar Parameter Value node, then the new Parameter Values will show up in the drop down. It can be cumbersome, but hopefully that will keep you going until we can investigate and implement a better solution to the issue.
For folks who have come across this before it gets fixed if you have a lot of parameters the workaround above doesn’t work. You have 2 options. Create BP variables for each param and wire them or you can add a bunch of blank collection parameters at the bottom of the collection parameter list. Keep in mind there’s only 128 per so once you get really full you have to do the wiring method.