Karoo 2 and wahoo HRM - no data at beginning of ride

have a Karoo 2 and a wahoo hrm not had any issue with with for last year but last couple of months (?) the hrm does not connect at beginning of ride but wakes up some time later. so today I have no data for first 30mins then hrm has connected and have the usual data. connection is via Bluetooth any ideas? Karoo has latest firmware