At any point during the time that you are in a forward, the spot price could move in your favour; if it does and the counterparty defaults or doesnt deliver the asset to you, you won’t see your gains materialize.
Lets say you entered a three month forward contract today on an asset at the forward price of $50 (i.e. you promise to buy this asset from the counterpart at $50 in three months). Over three months the asset price slowly went up to $80. This means that the value of your forward contract has slowly increased after three months (Spot price minus Forward price, 80 minus 50 = $30 gain). At any point during those three months, market conditions have changed, driving up the value of the forward which is in your favour (driving the value down for your counterparty - zero sum game). The forward is not traded on an exchange and hence is just a contract between you two, so if the PV of the forward is positive for you and the counterparty doesnt want to pay you are isnt able to pay you, you lose.