There's a lot of reasons why a bowler might not perform. Skills, fitness, stamina, match orders, bowling orders, the pitch and your opponents play the biggest roles. Your management is how you sneak unlikely wins when stats say you should lose.
Training takes a long time (probably the quickest popping is stamina at six weeks on 1 net) and is for the most part, affected by age. Your old guys probably aren't worth training except in special cases. Plus, training affects fitness. Some guys just can't train and play. Patience is the virtue needed here.
Consideration should be given to the pitch and your bowler's styles. Fast or fast-medium bowlers should take the new ball. You should have your seamers attacking on favourable pitches, but defence can important through the middle overs and also on unfavourable pitches. Bowlers with poor stamina struggle to bowl long spells and can struggle bowling at the end of an innings. Do you have many of one bowler type? This could be a strength if you have a home pitch strategy, but opposition will look to exploit it.