You may be able to play around with window functions (or apodisation as it's referred to in Mestrenova) to try and tease out this small coupling. Or it could just be that that proton relaxes faster.
I'm not 100% sure why, but one possible reason is because there is further unresolved coupling with the OH peak: instead of being resolved as a splitting in the spectrum this just broadens the individual lines. I suspect the coupling in the quartet is just not fully resolved.
