On a recent road trip,I noticed a strange issue with the CB

I was lead bike and was using it to stay in touch with the guy at the back. Although I could hear him clearly, my bike wouldn’t always transmit. At first I thought it might be an issue with the PTT button as I often had to wiggle this a bit to connect. However, after a while I realised that whenever I was riding at speads below 55mph, I could transmit but over that, pressing PTT had no effect

Any suggestions gratefully received!