One new isolated unidentified bacterium growing well on specific agar media but not on broth media of same compostion (except agar). What could be the reason?
You're going to have to give some more particulars. What temp & shaking are you using the for broth?
But this sounds suspiciously like an undergraduate homework question. Any real microbiology has an easy answer for you if you are actually trying to trouble-shoot a lab protocol.
As Katie A S Burnette mentioned, you have to be more specific. For example you have to check any kind of changes in temperature or other environmental parameters like oxygen requirement.
It might be due to different factors affecting growth. For example, pH is quite important. There are strains growing only within a very specific pH range.