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If a mast bends beyond what the mainsail can adapt to (beyond the luff curve built into the sail), the sail shape will distort and become inefficient.

See the drawings of typical over-bend creases and what the cause might be. Thanks to Ian Pinnell from P&B Sails

The way a 5O5 mast is generally rigged it is easy to get the mainsail too flat in the bottom and middle sections while having the top relatively full.

Too much bend in the middle where the spreaders are? The spreaders are too long for those conditions. Rake less or move shroud bases aft.

Too much bend at deck level? Pull the ram down to straighten the mast at deck level. it is easy to think “I need more mast bend I shall ease the ram. This is the opposite of what you need to do.

Too much Vang can also over-bend a mast.

Not enough power, particularly in the lulls? Make sure you have sheeted the jib on properly to ensure the top luff tell tale is flowing. Make sure yo have pulled the boom back to centre, eased cunningham and rig tension (and maybe vang).