First of all, we should really know whether you have really a weak serratus anterior muscles and in case would be interesting trying to understand why it is weak (neurological/functional wing scapulae?). Maybe you don't need any strengthening excercise at all, maybe the problem could come from a poor posture (e.g. rounded-shoulder) or from a hypertrophic/contracted muscle groups on your shoulder girdle. Unluckily from here is, without a proper evaluation, impossible to tell you what exactly you should do. I guess the best thing you can do is to go and have a chat again with the colleague, and let him/her know that the excercises proposed didn't achieve any particular good results and see if the management plan can be changed or modified.

All the best!