It sounds like Chlorosis may be what you're dealing with and is fairly easy to identify. In most cases, a chlorotic plant will be one that is highly active, gets a lot of light, uses water vigorously, grows rapidly, and is usually quite well-established. To accurately identify Chlorosis, look for the following symptoms:

• New growth on the plant grows in a very washed-out, faded, light green, or yellowish color that remains into maturity.
• The faded-out new growth will have dark green leaf veins.
• The older, established growth will be a normal, healthy, vibrant green color.

Chlorosis

If a plant is found to be chlorotic, it is not producing enough chlorophyll which allows for a plant to do photosynthesis. Chlorosis usually occurs when a plant is highly active and uses up all the minerals in its soil. The key mineral in deficit in a case of chlorosis is iron. The symptoms show up on new growth because the older growth has already established its chlorophyll content. When the plant attempts to produce new leaves, there is not enough iron available for the plant to create a healthy green leaf.

Correcting

Your raspberries will require a high iron fertilizer or soil additive. It is recommended that you try and find a fertilizer with chelated iron; this is much more easily absorbed and processed. If your plant is chlorotic, it is highly likely that it is deficient in other key nutrients and minerals as well, so ideally a well-balanced fertilizer with chelated iron would be the best option. Make sure to follow instructions well when introducing a plant supplement, but especially with iron; too much iron can burn and damage a plant; too much can be toxic.

Always be sure that your plant is in a stable condition when introducing an iron supplement or fertilizer or any kind. If a plant is overly dry or overly wet, it will already be in a weakened state. Allow it to come back in balance before introducing more variables that can cause additional complications.

NOTE: Any new growth that was produced in the chlorotic state will remain chlorotic (faded with green veins) even after the iron supplement has been introduced. If possible, this growth should be cut back for new healthier growth to take its place.

 Preventing Chlorosis

The plant should be given regular treatments of chelated iron-supplemented fertilizer or root stimulators. These help keep adequate amounts of nutrients in the soil and available for a plant to use in producing new growth.  For a quick and informative tutorial on chelated iron please refer to this website.

Best of luck and happy gardening.