I've just started a new map and ive used about a third of my budget but for some reason im unable to place certain objects like tunnels or curved walls. I can also only duplicate some items and not others?
I think there is a hidden limit of items to place if you use too many of certain ones. What did you use a lot of?
There are limits that apply to each category of items. They're not hidden, but they're easy to miss: in the Forge menu, look for a number next to the name of each item. That number is how many more of the item you can place before hitting the per-category limit.
I know in Reach there was an actual piece limit where you could have budget and pieces left and you wouldn't be able to place them. But with only using 1/3 of the budget I highly doubt that is what you hit. Most likely it is what DavidJCobb said.
I know about that item number limit, but there is some kind of "hidden" limit to how many pieces you can place on a map because I hit it on Erosion. I couldn't even put any spawn points down even though I didn't use any yet and I had the full number allowed still displayed. I was kinda close to maxing out the $10,000 though, probably around $8,000 I used.
650 item limit, no matter what item, you can only spawn a total of 650 items total. If you only hit 1/3 of your budget, I doubt you spawned 650 items.