I purchased an older Howard Miller clock and the clock is keeping time but pendulum is barely moving. What do I do?

If you need to ask how to fix it, it probably means you shouldn't be tinkering with it.

You have something that represents a fair chunk of money and it would be wise to consult an experienced horologist about your problem or perhaps consult the manufacturer.

The cure might be as simple as removing the hands and putting them back to coincide with the chimes but it is presumably a fault that developed over time. I doubt the hands moved their relative positions by themselves but rather something inside happened.

Owning a mechanical pendulum clock is a hobby that requires some basic knowledge about how it works and how to care for it. The hands should never be turned backwards, for instance. Changing the time for daylight saving means either stopping the pendulum for an hour or advancing it by an hour and pausing at each quarter to allow the chimes to sound fully. Any mistakes will mean winding on many hours or stopping the clock and waiting.

OK, google is your friend, but before I go there, recognize that pendulum clocks are generally regulated by the speed/period of the pendulum. A shorter pendulum has a shorter period/faster time and vice versa. So in general, look for the length adjustment of the pendulum weight.
Specifically, here is a link to a google-d search:
http://www.howardmillerusa.com/customer-support/how-to-setup-howard-miller-grandfather-floor-clock.html

When was the last time it was serviced? The movement probably needs a good cleaning and/or oiling.
UsedClockParts.com

Hi Pam,
I have a Howard Miller Grandfather clock and the pendulum swings very slowly and from far left to far right travels only about three inches at best. It doesn't need to move any more than that to make the "Verge" move the "Escape Wheel" to rotate the gears to keep the hands moving at the correct tick.
Howard Miller Grandfather Clock

If the clock has a real clockspring mechanism ( with dozens of visible gears), then the problem is likely it needs to be cleaned.and adjusted.

http://www.howardmillerusa.com/customer-support/how-to-setup-howard-miller-grandfather-floor-clock.html
Click on this link for the setup info you need and read

When was the last time it was serviced? The movement probably needs a good cleaning and/or oiling.

http://usedclockparts.com/8-clock-movement-repairs/39-how-to-easily-clean-and-or-fix-your-clock-movement

Just taking a stab that someone replaced the suspension spring with one that is too long, and beyond the pendulum adjustment.

On mine, the center one (@the 6 position) is the one that keeps the pendulum swinging. It is the one that lengthens the most during the week and if I forget to wind it, that pendulum is all the way down when the clock stops.