This happens in the beginning of chapter 27 of Great Hunt, where Rand and Loial are attacked by Trollocs in the Foregate.

 

When Loial is wrestling with the first Trolloc, Rand realises he can finish this fight with Saidin, but

There was nothing there. [...] The taint slid off onto him, soiling him, but there was no flow of light inside him.

(that's a quote from the books).

 

What happened here?

Others might have a better answer, but I always interpreted this as one of a few examples of Rand struggling to *learn* how to channel (i.e., he is not able to access and control Saidin at will at the beginning).

Yeah, Rand just hasn't learned yet how to use Saidin. This continues through TSR until he finally gets Asmodean to teach him. Learning on his own he struggles just like Egwene did back in TEotW. He talks about this again in TSR when Egwene and Elayne come to try and teach him.