Touscreen one part not working sometimes

It sounds like your connection for the touch screen is loose. The area where you are pressing is fairly close to where the connector comes from digitiser to the motherboard...perhaps you have dropped the phone? It may be worth opening the phone, use a guide linked under the photo to the right and examine connector 2 flex for any kinks or rips and that the connection onto the motherboard is snug. Sometimes it's worth "unplugging" and then replugging in the connector to make sure it is snug. Always work on the phone when you have powered the phone off using slide to power off, I'd also advise not to try testing screens etc while you fiddle about with the connections..it will short out your backlight etc.