flatbeer

Faulty closures on the bottles or worn out washers on the barrel. Check the condition and replace if necessary.
Temperature for the secondary fermentation is too low.
The beer needs to be kept warm for at least 5 days for the secondary fermentation. It it is in too cold a place, you will not generate the fermentation and pressure. Next time, keep the beer in a warm place (between 21C and 27C) or use a Heating Tray or Brewbelt for the first 5 days after the beer has been bottled/barreled. Then move to a cooler place.
The beer is stored in too cold a place.
After the secondary fermentation has completed, move the beer into a COOL place, not cold. If the temperature is too cold, the pressure will be lost. Bring the beer into a slightly warmer place (15C).
The time between fermentation and bottling/barreling is too long. Use a hydrometer to ensure the correct time to bottle/barrel. Leaving the beer too long in the bucket will not give sufficient pressure during the secondary fermentation and also higher the risk of infection.
Not enough priming sugar added.
Next time add 1/2 teaspoon of sugar per pint of beer before sealing the caps.