Have I broken the gearbox in the workshop?

Hello, I carried a Honda CB 250 motorcycle from 94 to the concessionaire because I lost oil and handed it to me once the \ "axis seals", but when riding on it, the marches did not come in, it had to be squeezed a lot especially to climb speed, in the workshop they tell me that they tried the motorcycle and they did not notice anything, that that is a new and fortuitous breakdown with which they have nothing to do. I have requested arbitration for consumption, I do not understand mechanics but for me it is clear that something has done wrong, but what could happen? How can I prove it even if it is theory? There is some manual that explains what are the causes of the malfunction of a motorcycle gearbox, in order to determine what happens to the motorcycle and how it has been caused. Thank you so much.