In Chrome, tap to look up in dictionary: Placement not working properly
The word needs to be in about 1 inch from top or bottom of the screen for pop up to work.  

It wont work on all type of web pages.

Here is a example of the pop up relative to the position of the word on the sceen.

The word "Bittersweet" is Up most position for the pop up to work



Mid screen



Low, in this case you actually can see the word in the sentence and the pop up is above it.