Can the range of a monostatic RFID antenna be increased using an amplifier?
Passive RFID works because the tag varies the amount of signal power it absorbs/reflects. This is seen by the reader as a change in the "radiation resistance" (i.e., impedance) of the space around it, which causes small variations in the reader's antenna current and/or voltage. The reader looks for specific patterns in these varations in order to receive the tag data.

The range is limited primarily by the magnitude of these changes relative to other changes in antenna power that are due either to other objects in the environment, or to amplitude variations in the transmitter circuit itself.

You can't simply add an amplifier between the circuit and the antenna of an existing reader, because this would prevent the reader from sensing the radiation resistance. The reader itself needs to be designed to operate at a higher transmitted power level while maintaining (or even increasing) its signal-to-noise ratio for the tag's incoming signal.