An important source of forgetting on working-memory tasks is the similarity of previously learned material. The finding that people may have trouble learning new material because of interfering effects of previously learned material is called

Proactive interference

Explanation:

Interference is simply the explaination one gives for forgetting long term memory. It occurs when a similar information gets in the way of an information one wants to recall or remember.

It is of two types, Proactive and retroactive.

Retroactive is when a much newer information prevents you from recalling a much older information.

Proactive is when an old information gets in the way of recalling a new information.

People who find it difficult to learn new materials due to interference effect from old materials learned are simply suffering from proactive interference.