RSS Feed not updating/refreshing

I have been updating my website which has led me to revisit this issue that has been bothering me for a long time.
The only way I cam manage to get fresh output from the RSS Feed Retriever is to manually change the number of items that the retriever is to display.
For example I change something like
[wp_rss_retriever url=” xxx” items=”12″ excerpt=”75″ read_more=”true” new_window=”true” thumbnail=”150″ cache=”21600″]
to
[wp_rss_retriever url=” xxx” items=”14″ excerpt=”75″ read_more=”true” new_window=”true” thumbnail=”150″ cache=”21600″]
Please advise how I might go about resolving this issue once and for all.