A.  Check your knives.  If they are dull, instead of trying to cut the material they are pushing the wood off.  This causes excessive vibration that will cause the head to lower.  The knives should be able to cut paper or your finger if you are not careful.

Q. I have an Oliver 4455, 22" planer and no matter how I set it, I can't seem to get rid of the snipe.

A. Really, unless you get a planer with double outfeed/pressure rollers, it is virtually impossible to eliminate snipe. When the material first enters and then leaves the planer, it is only held down by one pressure roller.  This single pressure point can allow the material to pivot up into the cutterhead, especially if your material is long and heavy.  It is possible to minimize the snipe by first setting the machine to factory specifications.  The diagram below shows where to set the rollers, chip breaker, and pressure bar with respect to the cutting circle.  Typically, a good rule of thumb for all planers, as described in the diagram is to have everything set to 0.032" of an inch below the cutting circle except the pressure bar which should be set to 0.004" above the cutting circle.  Again, this will help minimize snipe but won't eliminate it.

You can also adjust the height of the table rolls so that they are just a hair above the height of the table. Too much however, will allow for another pivot point as the material hits and leaves a bed roller.  Another think you can do is to attach some infeed and outfeed supports.  These supports should be set to say, 1/8" or more above the height of the main table.  This will want to push the material up thereby keeping it down solid on the main table.