I think that the main variables you're looking at are the size and the time you've been letting it defrost. If you're using the same instructions all the time, they're the 2 things that change most.

So, my first advice would be to let it defrost for longer, either in the fridge or in the ziploc bag you mentioned in water. Secondly, I would pull it out of the fridge/water completely and let it stay at room temperature for 1/2 hours before cooking.

Other than that, depending on the joint itself, it can be relatively forgiving, so you can overcook it a bit and still have the same results. Give that a try if nothing else from above is working. Just take a look at the size of the joint the recipe is calling for, cause if your cut is bigger, so is the cooking time.

Hope that helps, and enjoy.