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just to add to diy guy's comments... a few drops of washing up liquid in the mix works as a poor man's plasticiser. mortar should never be stronger than the substrate you are working with... in other words when movement occurs due to changes in temperature or moisture the mortar should give a bit instead of cracking the brick, tile, etc., or simply becoming detached. 4:1 represents a strong mortar for best brick, again depending what sand is used (using a softer or finer sand tends to produce a stronger mortar but a more pleasing pointing). softer or weaker substrate would need a weaker ratio down to 7:1 for some jobs. it is still important that ordinary portland cement is fairly fresh and in good condition for best results... it is doubly important that mortar is allowed to "cure" rather than just dry out. consider bricks are very absorbant - like sponges - it is not surprising that in the old days brick piles were continuously soaked with a hose prior to being laid during dry spells to prevent this problem. i suspect this is the basis, or the majority of eddief's problem with his weak bonding mortar.