The larger molecules of your moisturizer are designed for occlusion (seals in your serums and hydration) and to strengthen the barrier function/ protection of your skin from the elements, pathogens, pollution, etc. A common and tricky gray area would be Oils. Oils can be marketed as serums, yet oils are very occlusive and have very large molecules and at times are even thicker than some moisturizers.
This is where I stay true to my thinnest to thickest philosophy. If you're putting on a thick oil first and putting something lighter in consistency on top of it, the lighter product is most likely not penetrating through the oil. (If you are using a medicated topical cream, use as directed by your physician or by the manufacturer. Since medicated topical treatments vary for cause, consistency and strength, I can't give it a blanket statement on how to use them or when, but if you consult with me in a facial I can get more information from you and assess your skin to give you a more educated answer.) Did this guide help you? Comment below!