If I understand correctly you already had Bachelors degree in Computer science then almost 5-6 years of total expereince and you opted for college diploma course?
if thats is so then its clear refusal as any visa officer will think with such experience and profile why he is not going for masters level degree in university why diploma at college level. which means his intention is to stay not to leave? how this college level diploma will help him in his career where he already gained some industry work experience in similar/same area. in this case you need very strong SOP, strong hometies and so on..
try to opt for masters level degree.
That is my take out of it. other group members can give their insight.
Thanks