i The fibres may already be clumped together before they are added to the mix; normal mixing action will not break down these clumps.
ii Fibres may be added too quickly to allow them to disperse in the mixer.
iii Too high a volume of fibres may be added.
iv The mixer itself may be too worn or inefficient to disperse the fibres.
v Introducing the fibres to the mixer before the other concrete ingredients will cause them to clump together.