Heart rate recording inaccurate when pool swimming.

I've had my Instinct 2x solar for 5 months and used it regularly for tracking pool swims amongst other exercises. Lately the heart rate tracking just doesn't seem to pick up or takes up to 40 minutes into my swim before it jumps from around 80 to around 130bpm where I'd expect it to be. I'm wearing the watch as tight as it needs to be and haven't changed anything.

Any help/ideas please?