I've read through a lot of the posts, looking for anyone having this issue, but haven't seen anything. I've downloaded a number of things from here, and one or two other sites (some people are doing some very nice design work, btw. I hope I can do that well) I've had no problems downloading or getting the files into the appropriate folders (usually the Themes area, sometimes the Structure folder)
I've had no problems with any pre-made structures, nor with anything placed at ground level, ie statues, path covers, decorations, that sort of thing. The problem I'm having is with things like wall sets, girders, or any construction sets. I can place items at ground level, but when I go to stack them on top of each other, the height adjustment doesn't work. I've tried with 7-8 different sets, and none of them work. I have no problems with the preloaded sets, just any newly downloaded ones. I really want to expand my construction options. HELP

If you downloaded many the items you are talking about, I would send a private message pm to the creator of the structure you are using.

The height adjustment works by holding the "shift" key. This should be clear, or not?

If you can't stack objects like walls above each other, either the CSO is corrupted what i think it isn't (because corurpted cso's crash the game before placing) or your RCT3 or your PC/Laptop has some sort of damage.
I would re-install the whole game, while safing the "themed" folder before.
After re-install everything should be default.
Than i would copy only the "not working" cso from the safed themed folder into your new one and check it out if it now works in the game.

I never heard about errors like that, so i can't give you a clear solution for your specific problem.

So long, Cobra

The height adjustment works by holding the "shift" key. This should be clear, or not?

If you can't stack objects like walls above each other, either the CSO is corrupted what i think it isn't (because corurpted cso's crash the game before placing) or your RCT3 or your PC/Laptop has some sort of damage.
I would re-install the whole game, while safing the "themed" folder before.
After re-install everything should be default.
Than i would copy only the "not working" cso from the safed themed folder into your new one and check it out if it now works in the game.

I never heard about errors like that, so i can't give you a clear solution for your specific problem.

So long, Cobra

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I do know how the height adjustment works. Yes, it's quite clear. I'm pretty sure the CSO's are not corrupted. One, maybe. All of them, very unlikely. Like I said, all of the sets that come preloaded with the game work just fine. I've been building many structures, some quite tall... castles, pyramids, etc. The problem I'm talking about affects only the new sets I have downloaded. There must be something I'm missing while adding them into the game folders. I download, unzip to the desktop, move the folder containing the set into the corresponding game folder. If there's a readme file, I read it. I make sure not to move the readme into the game folder. I make sure the ovl files are present. As far as I can tell, I'm doing everything I should be doing. In all cases, the sets appear in the game, but the height control does not work. Not with themselves, and not when I try to combine them with anything preloaded. And no, there's nothing wrong with my computer. It is only 6 months old, has a quad-core processor and 8 gigs ram, and is in perfect working order. Do the ovl files get moved to the same folder as the items? Any chance they go somewhere else? There's obviously something missing, some little thing I'm not doing right.

If you change anything in the cso folders it can mess it up a lot. If you did that I would remove it and download it again. I hope I understood you correctly on what you were saying. If not apologies in advance. If you are trying to take a whole structure and add to it by moving it up one, you may not be able to do so without running into problems. It sounds like you know about the shift option in the game to raise the walls up individually. I hope you will get the answers you need . Not to sound like a broken record, but I would try and private message the creator of the files you are talking about to get more assistance.