Hi. Without more information it is hard to say what is killing your raspberries. I will tell you that root rot is the most common reason for raspberries dying in the Willamette Valley. Raspberries are very susceptible to root rot. Our heavy wet clay soils do not help the situation. To determine if this is the issue facing your raspberries check the roots. Roots should be white and firm if they are healthy. Roots suffering from the fungal condition that rots the roots  will be brown or black and mushy or will be totally gone. Check the plants that are dead, then plants that are showing symptoms that you describe and finally those that are not yet showing symptoms and compare them. Raspberries need to be planted in areas with well drained soils or in raised beds 12-18 inches high and watered carefully. There are some types of raspberries that are less susceptible to root rot.  The link below will take you to an OSU publication that will discuss types of raspberries and also includes a table part of which tells you which varieties are resistant to root rot. 

https://catalog.extension.oregonstate.edu/files/project/pdf/pnw655.pdf

If your raspberry roots appear to be white and firm I suggest that you address this problem further with your County Extension office’s Master Gardener information desk.  Benton County’s Master Gardener Help Desk phone number is<personal data hidden>.