My share sell price is lower than the price people are buying yet no sale?

I'm trying to sell some shares at a sell price of 0.5. I can see from the buy/sell history over the past few days that shares are being purchased for 0.6 and others sold for 0.4.

You are looking at the last trade prices. This is information from the past. What is important to you now is how much others are willing to pay to buy your shares now. For this, you will need to look at the current bid prices instead of the last trade prices.

See: Can someone explain a stock's "bid" vs. "ask" price relative to "current" price?

My question is why are my shares not being made available to the buyers paying 0.5? Is there something I need to do in my sell instructions?

Assuming you have placed a sell limit order: One possibility is that the current bids (i.e. the prices others are willing to pay for your shares) are all less than 0.50, and there are no market orders that have taken your 0.50 offer thus far.

Without a stock symbol and the details of your sell order, there's no way to know what's going on.

My guess at a reason why your order did not sell could be that you placed an AON order (All Or None) and not enough shares were available to fill your entire order so there was no fill at all.