I can give you a couple of 'feels' and hopefully that will help. Sounds like your rotation with your chest is stopping/slowing and that is what's causing the disconnection.
1) Left arm only swing: put the fingers of your right hand under your left armpit and squash them a little. Make a left arm swing so you feel that pressure is maintained on the fingers throughout the entire swing (after you've hit the imaginary ball) until late in the follow thru when your left arm hinges up - that is being connected. Your chest MUST turn and keep turning way past the 'hit' otherwise that pressure is lost thru impact, then the arms disconnect and fly down the line rather than around with your chest in an arc.
2) Imagine that someone is standing to your right side and THEY are reaching across the front of your chest and they have their hand under your left armpit and they are pulling you into your backswing and then trying to STOP you making a downswing turn by holding onto your armpit... that resistence is the pulling feeling you should have with your upper body all the way thru the ball with your left arm 'pressurising' your left armpit... you can let the left arm fall down your chest into impact but the pressure in the left chest/armpit remains.
A two plane swinger or someone who throws the club down the line (or a caster or flipper) won't have these feelings because they can't pressure the armpit for the whole swing whist swinging their arms... the armpit pressure is lost and they rely on some timing/flipping/rolling of the club into impact to square the face rather than the feeling of the clubface being an 'extension' of the armpit (eg: when the armpit turns the face will square).
