I can't add additional items to the Office Clipboard.

There are already 24 items on the Microsoft Office Clipboard   If you copy more than 24 items, the Office Clipboard will delete the first item copied and then collect the 25th item. Collected items remain on the Office Clipboard until you close all Office programs running on your computer. To begin copying items again, delete some items or click Clear All on the Office Clipboard.

There are large items on the Office Clipboard   The Office Clipboard may stop adding items — even if there are fewer than 24 — if you copy large items to it. The Office Clipboard can contain up to 4 megabytes (MB) of data if system RAM is less than 64 MB, or 8 MB if system RAM is 64 MB or greater. To begin copying items again, delete some items or click Clear All on the Office Clipboard. If a single item is larger than 4 MB or 8 MB, you cannot copy the item to the Office Clipboard.

You are attempting to copy an item with an unsupported format   There are some items that cannot be copied to the Office Clipboard because the format is not supported.