The cause of the problem could be that the key is unable to shift the levers properly. There are two main reasons why this happens. Firstly, dirt and debris accumulate in the keyhole. Secondly, keys wear out over time and the accuracy of the cuts decreases.
In both cases, the shape of the key doesn’t match the levers accurately resulting in more force required to turn the bolt. It is advisable to avoid applying excessive force in such situations. Lubricating the keyhole or using a new key should solve the problems. If the issues persist, contact professional personnel.