Laminectomy is the removal of part of the vertebral column that forms a circle around the spinal cord. The idea is to take pressure off the spinal cord or the spinal nerves leaving the spinal canal.
In order to get to the bone, the surgeon must cut through the muscles and ligaments on alongside the spine. If the surgeon can't see the area well enough to complete the decompression, then part or all of the facet joint is also removed.
There are several reasons why you may not be able to regain your full strength. There can be atrophy of the paraspinal muscles on either side of the spine. Local nerve damage and loss of support from the facet joints can make a difference, too.
An MRI might be helpful to show a cross-section of the muscles to look for atrophy or wasting of the muscle fibers. This is more helpful if you had an MRI done before the operation to compare before and after.
Depending on how far you want to pursue this question, you could also talk to your doctor about EMG studies. Electrodes used to pick up electrical signals from muscles can be measured to look for abnormal or weak patterns.