There are many reasons for bug holes in concrete but 3 specific ones which I will highlight in detail.
Aggregate consistency: -
It is important, especially when making concrete paving slabs or blocks to achieve a smooth finish.One of the main issues for bug holes is that the user is using different sizes of aggregate, we recommend using stones from 6mm to 2mm (otherwise known as quarry dust or three-eighths). Using this size aggregate, we advocate for our products a mix ratio for the paving blocks which are 30x6x30 cm of 5:4:2 stone, sand cement. You want the concrete mix to be fluid for paving blocks to limit surface voids/bug holes. There are alternative mix ratios we use for paving slabs 45x3x45cm of 2:3:1 make sure you regimentally stick to the mix ratios for whatever product you are using.
By using a consistent mix ratio, it will help reduce/ eliminate surface voids, but there are other factors to consider.
Too Much Release Agent: -
When making formwork items such as waffle slabs, hollow blocks using a steel frame which we call casting moulds which are filled and turned out through the day we recommend engine oil sometimes mixed with paraffin to demould steel blocks. A thinner wax based agent is even more ideal to achieve a quality finish but more expensive. However, for making paving blocks or slabs, we don't recommend using anything other than water . (this is the case for all plastic setting moulds)A release agent must be used to demould the slabs. Otherwise, the concrete will stick to the mould, and you will have a tough time demoulding it (like trying to remove prey from a predators mouth) usually release agent is applied by a paintbrush all the way around the inside of the mould. By applying too much release agent, you may darken the mould and the product and from testing, we have seen increased bug holes from using engine oil so we don't recommend it for decorative products. You can purchase release agents from us.
Improper Vibration: -
This has never really been a problem we have experienced in our product creation but it can happen. When you are making our products on our vibrating tables we typically half fill the moulds with concrete vibrate to settle the mix and then fill the mould and vibrate until we see the water level rise to the top of the mould and then stop vibrating. Effectively if you don't vibrate the mix enough the air bubbles will become trapped between the mould and the surface of the concrete with a fluid mix you can reduce the amount of air and water bubbles by increasing the mixing; time.
Useful sites for further information: -
Portland Cement Association http://www.cement.org/learn/concrete-technology/concrete-construction/bugholes
National Precast Concrete Association https://precast.org/2014/06/causes-fixes-scc-bug-holes/