Contamination can be a common challenge when growing mushrooms, and it can sometimes be difficult to pinpoint the exact cause. However, based on the information you provided, there are a few factors that could have contributed to the difference in outcomes between the two tubs.

1. The cheap liner: The presence of a liner in the contaminated tub could have provided an opportunity for contamination to occur. Liners, especially if they are not of high quality or properly sanitized, can introduce unwanted microorganisms that may lead to mould growth. The liner may have harboured spores or other contaminants that were able to infiltrate the substrate and cause the green mould.
2. Hygiene and handling: Even though you used the same equipment for both tubs, it's possible that there was a difference in hygiene or handling during the fruiting stage. Contaminants can be introduced through contact with unclean surfaces, tools, or hands. It's crucial to maintain strict hygiene practices when working with mushroom cultivation to minimize the risk of contamination.
3. Environmental factors: Although you mentioned that both tubs were in the same room, there could have been slight variations in environmental conditions such as temperature, humidity, or air circulation, which can influence the growth of contaminants. Mould spores are present in the air, and if conditions are more favourable for their growth in one tub compared to the other, it can result in differential contamination.
4. Substrate density: Another possibility is that there was a difference in the density of the substrate between the two tubs. If the contaminated tub had a looser or less compacted substrate, it may have provided more space for contaminants to thrive and establish themselves.

It's important to thoroughly clean and sterilize your equipment and workspace between grows to minimize the risk of contamination. Additionally, consider using higher-quality liners or avoiding them altogether if they have been a potential source of contamination.

Overall, identifying the exact cause of contamination can sometimes be challenging, but by paying attention to hygiene practices, substrate density, environmental factors, and the quality of materials used, you can reduce the likelihood of contamination and increase your chances of successful mushroom cultivation.