First thing first, we suggest you to check that tracks, wheels and pickup are clean and that there are no dust or dirt residues.
If it is a digital model with an ESU decoder, it is probably necessary to perform auto-tuning to adjust the decoder according to the engine.
Switch off F1, set CV 54 = 0, put the model on a straight track of about 1.5-2 meters, and switch on F1.