Help me understand EM Field behavior around Ferrite sheet
I have to disagree with the route some of the comments are taking. I have used and continue to use ferrite sheet to enhance magnetic coupling between coils. I also use ferrite sheet to reduce or reshape the magnetic field so that eddy currents are not induced in structural metal parts.

Having said that, it depends on operating frequency and type of ferrite sheet that is used but, at the fairly common frequency of 13.56 MHz (used for a lot of RFID applications). I use Ferroxcube 5T1 materials - they have a magnetic permeability of 10 up to 100 MHz and they work just fine.

However, as with any tuned circuit system, if you place the ferromagnetic material close to a coil you'll shift the resonant frequency and suddenly find that it needs a re-tune. That is what I believe is the problem the OP has found.

Wurth also make this type of ferrite sheet but it looks even better at 13.56 MHz - material 364 has a permeabilty of 120 and seemingly very low losses too. Looks nice for RFID applications and, if you read the data sheet they tell you how much it can restore the tuned frequency when metals are close by. The clear inference of this is that it can also de-tune!

Here's what Wurth say about it: -



Note the table across the middle of the sheet and how inductance rises from 8.4 uH to 14.2 uH with more ferrite sheet thickness. A shift of 69% in inductance is a shift in resonant frequency of 30% and might take 13.56 MHz down to 10.4 MHz.