You may not have done something as right as you could have. One of the most important steps in changing cartridges and stems is properly preparing the faucet for the stem by removing any mineral buildup or debris from the faucet body. It doesn’t take very much to keep a new part from making a good seal. Take a cleaner formulated to remove mineral buildup and a small brush and really clean the faucet well, then try installing the stem or cartridge again.