The main problem they had (and this is sort of a recurring theme for Marvel) is that the real top talent departed the company who started filling their shoes with somewhat less skilled creators doing the same characters. The war draft for WWII was the main reason back then, of course, but Simon and Kirby were baited off by National/DC, leaving brilliant strips like Cap, Vision, Mercury/Hurricane, Tuk to the hands of the lesser lights that Martin Goodman could reel in.
I mean, Syd Shores was an okay artist, but he was not Kirby, you know?