There are several web sites like Flight Radar which depend in input from ground stations for their information. Many such ground stations are enthusiasts using readily obtainable equipment such as SBS who feed the information over the internet to FR. These simple gadgets receive secondary radar transmissions from aircraft (some aircraft, not all) and, using clever software, plot the positions on a PC screen producing displays not unlike ATC Radar. If the "ground station" happens to pop down to the shops or goes on holiday the coverage of quite a large area can be lost if the SBS is turned off. I have an SBS device with a small aerial on the side of my house near Reading, Berkshire. As I type I can see an Irish airliner at 38,000 ft over Liverpool. If I moved the aerial a little higher I could probably see over west Wales. However, I don't have SBS on all day so I do not contribute to FR or other sites. These sites cannot be relied on for anything like 100% accuracy.
Contrails crossing are not unusual because the airways cross. E.g. traffic heading from Scotland to Spain may well route south over your area as will "crossing" traffic from London to/from the USA. There will inevitably be military traffic which won't be "seen" except on real radar.
It's difficult to know what to suggest to help you further as there are so many variables.