I have an old mold, and it is becoming increasingly difficult to achieve a good releases.

Clean the mold thoroughly to remove any oxides, old releases etc. Make sure you seal the mold with additional THIN coats of mold sealer, for composite tools, you can apply up to 6 or more coats. Avoid applying lots of sealer at one time, or the temptation to pour the sealer on the mold and let it dry without wiping off. Remember applying multiple thin coats correctly, wipe on / wipe off, will be much more effective. Release initially with 2 coats of mold release, and make sure to apply "touch up" coats of mold release more often. If the mold is porous, cleaning and applying additional coats of the mold sealer, is a lot more effective than building up additional coats of mold release, or trying to over apply the mold release and (maybe) not wiping the product off correctly.