There are a couple possibilities:
1) There is an airflow obstruction in something that needs cleaning. The screen and mouthpiece can obstruct airflow and create performance problems. Clean or replace these parts, and make sure the pipe and top are not clogged. Run a pipe cleaner through the pipe, or clean it with our custom-made brushes, available here.
2) The filter might be clogged with soot. Soot can be a problem if it is inside the ceramic filter and not visible on the top surface. A ceramic filter that is partially clogged with soot will not work as well. See this page for how to remove the ceramic filter and burn-off soot deposits. Contact us here if you would like to buy a new top or replacement ceramic filter.
With proper use, VG filters will last for many years and will never become clogged with soot. DO NOT touch the flame to the ceramic!