My mum does that too. I sometimes adjust it so she is tilted upwards slightly, which can stop her slipping down, this might sound counterintuitive, but if her feet are flat on the floor this can stabilise her. I find it’s usually because she does not feel enough support from the back rest, maybe a firm cushion will help (although my mother refuses this!)My mother keeps sliding down in her recliner is there any thing I can buy to stop her sliding?