Not blocking shots

This is becoming a real source of frustration for me. We just keep letting teams take shots at us with no/half-hearted attempts to block them. It’s happening a lot, but it was especially noticeable against Boro - ( the losing goal came from it) - and other away games.
We let Watford have a lot of free shots at us first half - they just kept missing.
All 3 goals at Southampton were shots outside the area with no pressure on the ball. 1st one Doyle turned his back, 3rd one Klarer was in position to block but made no attempt to do so.
I’m not sure why this is - lack of fitness might be one reason, but I’m casting about really. Whatever it is it’s costing us, and I would like to see us improve in this area.
We are far too easy to play through the middle of anyway, but what I’m talking about here is us having players in position to engage/block and not doing so