Attention: Before you commence any electrical work, please be sure to turn off the power supply or consult a qualified electrician to seek further advise.
First you need to check if the wiring was done correctly: be sure that every wire is in the right position (as shown in the picture).
Be also sure of the origin of every wire: sometimes the colour doesn’t reflect the standard wiring code and this can be the reason of the issue.
Once you’re sure of the wiring correctness (all the wires are in the right position), if the problem recur it can be that the neutral wires of the two wall boxes are connected to two different electrical circuits. To do a 2-way application be sure to be using the Neutral of the same circuit for both switches.