What it probably means is .....
is that you are probably not raising the mainsail far enough. Most non-racing sailors hardly ever fully raise their sails correctly.Assuming that the sail is made from woven dacron, raise the sail as far as it will go. Then take a *square* of paper or cardboard put it on the sail side of the boom and check the angle that the boom makes with the mast .... it should be 'very' close to 90 degrees (usually 88 degrees but close enough). If the boom aft end seems to be much lower than it should be and the boom not at ~90 degrees ....... apply MORE halyard tension to the mainsail halyard until the boom is near 90 degrees. If you cant get 'near' to that 90 degrees, it means that the heavy rope inside the sleeve at the forward part (luff) of the sail has shrunken in length .... fixable by sailmaker. A sail in which that 'rope in a sleeve' is not shrunken, unless stretched out will make the boat s-l-o-w, will have excess heel, and will usually have a great amount of 'weather helm'. The 'general rule' for a woven dacron sail with a 'rope in a sleeve' (boltrope) at the luff is that that 'rope' after the sail is 'up' needs to be additionally stretched (by the halyard) by 1" for every 11 feet of luff length. Properly tensioning that 'rope in a sleeve' will 'raise the aft end of the boom', position the 'point of maximum draft' ... where the draft is the 'biggest' exactly where the sailmaker designed it for sailing in ~15kts of wind, plus the aft (leech) section of the sail will be 'cupped'. Putting correct main halyard tension will make that 'point of maximum draft' go further forward (towards the mast) and the leech portion will be come more 'flattened'. If that 'rope' isnt stretched out properly you will have a very 'baggy' sail, the leech portion will be very curved, and a boom that 'droops' (aft aft end will be 'much' lower than the boom connection at the mast.hope this helps.