Generic Tretinoin is usually as effective as the name brand. The next time you pick you pick up your prescription, talk to your pharmacist about this. Also, be sure you are using the correct strength. Retin-A comes in 3 strengths for a reason. You start with the lowest one and work your way up to the highest strength.
At this point in time there are many, many versions of Retin-A on the market and many makers of the product. Some are gels, some creams. The brand name isn't important. The tolerability of your skin with the product is what you need to worry about.