RCT2 Scenario Editor Question

So I'm trying to start a new custom park in RCT2 and am just in the scenario editor. When I get to choose which scenery objects I'd like to include, the majority of the custom scenery I've downloaded that I try to select just gives me a message when I click on it that says "Too many objects selected! Not all objects in this scenery group could be selected". Then, when I play the game, there are a few random objects from each category of scenery I picked. Does anybody know how to fix this problem? I have no idea. Thanks!

Select fewer objects. That's the only solution I know of.

Even if I only select one type of scenery, the message still pops up. Is there any way to actually select objects within each type of scenery?

Yes. Use the "advanced" category. Literally EVERY single scenery piece will be there for you to choose.

^That only works for scenery that was included in the game, but I'm having the issue with custom scenery. I can only get a couple objects from each category and the "advanced" button doesn't recognize any of it.