I suspect that like nearly everything else from the AoL, Shielding probably was something that was taught in a way that made the individual as effective as they could be with the weaves available. Perhaps there were shield weaves that incorporated other (or even all) flows of the Power that allowed for those weaker in Spirit to still be effective at shielding. I could see how earth, air or water could help strengthen a shield and how fire could assist with cutting off someone already holding the source. I always felt like AoL training incorporated assessing an individual's flow strength and weaves were taught that would maximize said individual's effectiveness with any given area of OP use. Thus a man very weak in Spirit might still be able to shield if he as taught a weave that incorporated fire or earth.

IMO this plays into the Aes Sedai losing so much knowledge. Eventually training was made into a "one-size-fits-all" class which eventually lent itself to only those with the strongest potential or an obvious Talent could excel and effectively perform the specific weaves required to become Aes Sedai. Modern Aes Sedai seem to only know one or two weaves for any given effect and those who are weak simply accept that they cannot move forward with these weaves.