Why is that sometimes I can not Paste into the Update pages input text boxes?

First, if you are Pasting a large amount or the input text box already contains a large amount, the Paste might exceed the text limit. If you are Pasting an amount that will cause your browser's limit for an input text box to be exceeded, then NONE of the text to be Pasted will be added to the input text box.

Second, if what you Copied contains a graphic or a chart or some other non-text object, then your browser may reject the whole Paste. You would have to first eliminate any non-text objects from what you copy. Generally speaking, Copying HTML is OK.