Troubleshooting Kindle chooses which Italian dictionary to use

[evwool]

I have 2 Italian English dictionaries on my Kindle; the Merriam-Webster and the (superior and half the price) Lucas Nicolato's Italian English Dictionary. Although I have successfully set the Nicolato Dictionary as my Primary dictionary on previous occasions, suddenly, even when I set it, when I open an Italian book, the Merriam is the one that displays its definitions when I hover over a word.
In fact, the only way I can get the Kindle to accept Nicolato is to delete the Merriam. Any ideas why this is happening?