A concrete wall blocks RFID signals only in some cases. It depends upon the thickness of the wall. The thicker the wall, the more signals it will block. Also, if metal-related material is used in the wall, then it will definitely block the signals. It is because of the fact that water and metals become obstacles in the transmission of electromagnetic fields. In the case of thin walls, RFID signals easily penetrate through a wall.
In some cases, it may also happen that the RFID reader can read all the tags, but if there is a wall in the middle, then their accuracy can be affected. The solution to this problem is to reduce the sensitivity settings of an RFID reader. It is done to shorten the reading range of a reader. Besides concrete walls, there are many other factors that determine the rate of transmission of the radio waves, such as frequency rate, type of tags used, the orientation of tags, etc.