Can't access s3 buckets after creating instance using IAM profile

I am trying to access some s3 buckets from an instance created applying the IAM profile that allows full access to those s3 buckets. I am able to carry out the required operations using aws cli. However, my application is written in Ruby and makes use of fog gem. Using fog, I am unable to access those buckets. All I get is Access Denied. Additionally, I grabbed the required keys: aws_access_key_id=`curl http://ift.tt/1diJPbH} | grep AccessKeyId | cut -d':' -f2 | sed 's/[^0-9A-Z]*//g'` aws_secret_access_key=`curl http://ift.tt/1diJPbH} | grep SecretAccessKey | cut -d':' -f2 | sed 's/[^0-9A-Za-z/+=]*//g'` and tried to access the buckets. Again, access denied.

Is there anything I should have missed?