Just because the property is old does not mean that you will automatically find old items,i am afraid it does not always work like that,when i gain permission to a old site i then do my research on the site using say Google Earth and other research tools to find the history of the site,this could be say near a old roman road or a old saxon settlement but that does mean will will find anything.
We went detecting on a farm a few year back on 120 acres but parked on a small 1 acre site across the road from the farm,the small 1acre field produced 100 times more than the farm did,the reason why the small field was a small medieval market and alot of hammered coins had been dropped in that field,also i found George 111 gold Guinea last year on the same site,so just shows that some in theory good sites can produce nothing and other sites you would in theory say had not got a chance turn up some fantastic finds.
You may try swinging slower,use prehaps a slightly bigger coil,the ground may have been dry as a result of a a hot summer but could come alive with some nice finds after a down pour,these are the types of thing that can change what appears to be a lack lustre site into a good one,of course if nothing is in that location then you wont find anything full stop.
