Extending UK Visa

Hello, I am a Russia citizen, but I live in the US and have a green card. I applied for the general visitor 1-year visa in October 2015 and I received my Visa for half a year only. I visited the UK in November and I just found out that I might be going this summer for a week or so. Is there anyway to extend my visa or is there any explanation to the fact that I paid for a 1 year visa, but was given half a year only

Thank you in advance :)