Why do my limit orders not complete?

Iā€™m getting very confused with limit orders. It might be related to it being a low-volume stock, but even then thereā€™s something else not right, as my buy order is being ignored when buy orders from other people at a lower price are being fulfilled.

After having been burned by placing a regular buy on TM17 (all the recent trades were around Ā£7.30), mine happily went through at Ā£7.50, I decided to test out limit orders.

I placed a limit order for Ā£7.30 and that completed immediately. I was quite pleased.

As the price had dropped to Ā£7.24, I placed another limit order for Ā£7.22 and went to lunch. Came back to find that there had been 4 trades lower than my buy price (all for Ā£7.20 or lower). As the price was now below Ā£7.20, I deleted my limit order and placed a new one at Ā£7.20 which completed immediately. Even though, Iā€™d just had an active one for a higher price moments before.

Iā€™ve had another limit order for 2 shares at Ā£7.12 placed for about an hour now. There have been multiple trades since then at Ā£7.10, including one for exactly 2 shares (which occurred some time after I placed my limit order), followed by some bigger orders 5 minutes later, also for Ā£7.10.

So, my questions are:

  • Why wouldnā€™t my limit buys succeed when Iā€™m offering more than someone else for the same quantity?
  • Why if I cancel my limit buy and create a new one at a lower price, will the buy complete even though the original one didnā€™t?

BTW, Iā€™m using the history on Google Finance to see prices & volumes of trades.