There are several options for that. If your emonTx uses a RFM69CW transmitter module, which it will if it’s reasonably new, then you can increase the r.f. power.
The second is to put a sheet of metal acting as a ground plane underneath both transmitter and receiver. There was a lot about this some time ago - Improving RF signal strength emonTH emonTX
The third is to move either or both ends just a little - that can make a surprising difference if one happens to be at a place where the reflected signal partially cancels the direct one.