Parallel transmission of spokes pulses is a promising way of mitigating flip-angle inhomogeneity in 2D imaging at ultra-high-field MRI. Bipolar slice-selective gradient is often used to minimise the overall duration of these pulses, making them more resilient to off-resonance related artefacts, but they are prone to errors caused by RF-gradient timing mismatch. In this study, we present a mathematical description for the effect of such delay on bipolar-gradient composite excitations. We demonstrate the effect with both flip-angle maps and EPI images. Finally, we propose a navigator approach to estimate the delay and show two effective ways of eliminating these errors.