iPhone 7 wont automatically reconnect to car stereo via Bluetooth

my iPhone won’t reconnect to my car stereo automatically via Bluetooth. iPhone 7 and pioneer MVH-S410BT. I ha e paired the devices and they work fine, then when i turn the ignition off and on again the stereo searches for the iPhone but can’t find it ( Bluetooth is always on and set to auto connect). I try to connect via my iPhone but get the following message “ connection unsuccessful. iPhone can no longer connect to MVH-S410BT. Forget this device and pair it again”. So I have to go through the whole “forget device , pair device “ every time. Can anyone help so that the devices automatically pair? Thanks