You have an inventory cap in Mystery Manor. You can see how many items you have at the top of the screen - the little bag icon shows how many you have and how many you can hold total.

Try not to let your inventory fill up, as any further rewards are wasted. If you are approaching your limit, simply use some items to fulfill desires or use them to craft the more complex items.

Of course, you can always upgrade your inventory using special items. For the first upgrade, you need two metal. The items will change between levels, so it is always good to work towards upgrading your inventory.