majah,
I'm going to assume that you're using UPLC rather than HPLC as you said you're using Marker Lynx. For most UPLC datasets I've seen I set my parameters as peakwidth=c(3,20) and depending on what instrument you're on the mzwid of 0.01 is a little low. It might be worth checking the raw peak list to see if the peak is there before grouping and in how many files. Using something like the following code:
## assuming your object is called xs, i.e. xs<-xcmsSet(....)
head(xs@peaks)
idx<-which(xs@peaks[,"mz"] > 264.14 & xs@peaks[,"mz"] <= 264.10 & xs@peaks[,"rt"] > (0.28*60) & xs@peaks[,"rt"] < (0.3)*60)
length(idx)
xs@peaks[idx,] ## this will show you how many files the peak was found in and if it was found.
If it finds the peak, which it should then you want to look at your groups/boxes manually to check you're not grouping 2 or more features into the same box (i.e. sleep >0). If this does start to happen I would check out the nearest method for the grouping.
?group.nearest
gxs<-group(xs, method="nearest")
Should do the trick Hope it helps and let us know how you get on.
Paul