I think you need to set the "guage" option. By default MRTG is to meaure the increment between an ever increasing counter. In the context of a network interface this works as follows:
time0: 1000000 bytes
time(0+5): 1500000 bytes
MRTG takes the difference between the two (500000 bytes) and then says that there was 500000 bytes transferred in that 5 minute interval.
So in what you have done it is seeing:
so it takes the difference between the two, which is ZERO.
read the doc at: