Head (comode) Not holding water?

This is usually a quick easy fix.  Let's address the first possible cause.

Most heads have a separate tank designed to add de-odorizers at each flush.  These are usually located either in a closet in the bathroom or under the vanity sink. If it is empty they allow the head to suck air thus not allowing the head to function properly.  The fix at this point is either refill that container or pinch off its supply hose.  You should see a plastic device located on the hose coming from the tank. This device has a self locking feature that allows you to control the amount of fluid delivered at each flush. You can open the flow or shut it off. 

The second possible cause on most heads is a stopped up vent valve.  Look at your head and if you see a retangular area framed in behind the head. It usually rises above the head about 6 or 7 inches.  It also will be approx. as wide as the head itself.  Look for screws that attach it to the wall. Sometimes these screws have a decorative cap over them. If so using a butter knife pry the cap screw cover off and remove the screws. Simply at this time slide the box upward and set it aside. You should now see a bent tube attached to the wall with hoses attached to each end.  On the top of this tube is a plastic fitting. Using a pair of pliers turn it counterclockwise and remove it. There is a rubber duckbill vent there that is most likely stopped up with corrosion. Remove it from the cap screw and carefully clean with dishwashing liquid or obtain a replacement valve from the manufacture.  P.S. This is a common problem so it's nice to have a few on hand.

The third possible cause is the run time adjustment after the initial flush.  This is actually a water saver switch for lack of better terms. Some units actually have a second small switch at the flush lever that can be moved forward or backwards for different levels of water usage.  Newer heads have a control board mounted underneath the head.  PLEASE NOTE that if this switch is mounted under the head and used to work fine this is most likely not the problem.  If it's always had low water levels you will need to remove the two floor mounting bolts so to allow you to move the head out and around for inspection.  There will be pin selectors that you can re-position to solve the problem. PLEASE CONSULT YOUR OWNERS MANUAL for this identification and selection method

Reinstall in reverse order and problem solved.

Uncle Ricky says it's as simple as that.