This is usually caused by a clogged air flow. Start with making sure the filter is clean.
Check behind the filter, at the inport hole. This is a common place for clogs to form, then back up into the hose.
Detach the hose at both the tank and powerhead. You may be able to see a clog at either end. If not, hold one end up and drop a coin in. Lift hose so coin drops through. If the coin doesn't exit, there's a blockage. You can clear it by using a broom handle to push it back through.
Check the powerhead where the hose attaches. If a clog isn't visible, drop a coin in the opening. If it doesn't fall through, the clog can be cleared by using a wooden spoon handle, or similar blunt object, to push it through to the brush and out.
For additional troubleshooting, see the manual located below.
http://www.eureka.com/service/manuals/77418_r2.pdf