Even in the center of a large thermal, if you lose contact with the strongest lift, you probably have been ejected by the turbulence of the core. When this happens, it usually will push you out the downwind side as your thermal boils up through stronger horizontal winds at higher altitude. New lift rising from the same source will arrive upwind of the position to which you have drifted. So in the absence of any other indication, and especially if a climb seems to have slowed with unexpected suddenness, moving upwind will often bring the climb back to life.
While advancing to more aggressive techniques, be sure you fully understand the fundamental aerodynamics involved. As an aircraft is banked steeper, more up elevator is necessary to raise the angle of attack and maintain slow speed – and you need more opposite aileron to prevent rolling into a steep spiral. Also, slower speeds require more rudder when steepening or flattening a bank. Strong or narrow thermals often demand small, short-term adjustments to the airflow in all three axes of control in order to achieve a round circle flown with constant attitude.