The automatic beatmatching in Mashup works well for most songs, but occasionally a slight adjustment can make things even tighter. If you are running into trouble, try rounding the BPM on the track controls to a whole number. For example, if you track A that is reported in Mashup as 127.89, and track B that is reported as 126.78, try entering 128 for track A and 127 for track B. This will fix most issues with beatmatching, but if things still sound a little off, try making a phase adjustment using the Phase buttons on the track controls.
Will you please let us know if that helps?