The webcam should not be “pushed forward or backward”, it should be rotated with respect to the right axis.
You have to follow the instructions and the pictures of the instruction files that you download from this page:
https://www.theremino.com/en/downloads/automation#spectrometer
In particular the following PDF:
https://www.theremino.com/wp-content/uploads/files/Theremino_Spectrometer_Construction_ENG.pdf
On the first pages you can see how the webcam must be mounted, and in the page 10 you can see how it must be rotated about 30 degrees.
Furthermore, be careful that it must be rotated on the right plane and that the rotation should be modifiable if the spectrum is too far to the left or too far to the right to be able to perform the software calibration operations.