Mine stays up fine (Queen) with blanket and comforter on it. Sounds like the struts are worn out. When the bed is down, the struts should be nearly horizontal, so they don't exert much upward force on the plywood base
I've used these guys for struts on the busses I maintain - their prices are good, and shipping is fast
http://www.spdhardware.com/catalog/l...-4E2E52A3BDA2}
They can help you pick the right struts for your application