Most beginner players often miss out on serious value (money) during the hands they play. Even some regular veterans are prone to missing out on value and their win rate is suffering because of this 'leak'. To ensure we make money in poker, we need to constantly be getting money in the middle with our value hands. This means making good sized bets and getting worse hands to call us.
We also need to build the pot on each street with our strongest hands so that we don't miss out on future value situations. For example if we opt to slow play and check the flop, it means we're missing out on value since we'll find it tough to get all the money in the middle by the river.
Where we can miss value:
1) When players decide to slow play.
Slow playing can sometimes be a great play but generally we should be doing the betting with our strong hands. Continuation Betting on the flop is standard, and so is double barreling - If you're playing solid poker you should always bet your strong hands, many times your opponent won't believe you have a monster.
We should only be slowplaying when it's going to work perfectly. If we know that the villain will call our check raise and bomb off his stack or we know that our opponent will bluff every street and let us call down, then absolutely we should slow play. However without this information, we should bet our strong hands as standard.
We sometimes miss value from slowplaying as we feel the flop is wet (possibility for many hands and drawing hands) so we want to check raise. This will often backfire if our opponent checks back, because if a scare card comes on the turn we'll have no idea where we stand.
2) We can often undervalue our hand and slow down when a scare card falls on the turn.
We need to put our opponent on a range of handsand play accordingly. Let's say we're opening JJ, we cbet the 472 flop then check the K turn once called. We are more than likely up against smaller pairs and now missing value vs. them and also some combo draws. We need to get value/protect our hand.