Before you start adding boosters double check the integrity of your bus &/or wiring.
Easy enough to do using a simple multimeter/voltmeter & a "load" such as a 12v 21w indicator bulb.
Connect the bulb (rememberb that it will get hot) in parellel with the meter (set to, IIRC AC volts 25v + setting)**.
Check & note the voltage as close as you can to the output of your controller, keep taking readings round the layout - that will pinpoint any defective joints/connections. If all proves well, then look at the possibility of a heavier bus to the far side of the layout.
** Forget the "can't measure DCC voltage with a basic multimeter" guys, what you are looking for is consistant readings with a load (you can also forget the "short out the track with a coin" method too - the controller will probably trip before any dry joint heats up).