Typical loss off vacuum is caused by air leaking into the surface condenser through the joints or packing glands. If you have an air ejector, make sure its assisting with drawing proper vacuum. Most issues related to the air ejector can be attributed to the nozzle errosion, the strainer protecting the nozzle is clogged, or the steam pressure to the nozzle is too low.
What are the temps of the seals on the LP end? hotter than normal temps could enlarge clearances and effect the seal. What is the rate of decay from the normal operating vacuum? What type of turbine do you have?