Continual improvement in therapy is the exception, not the rule. Stalls and deterioration may indicate a problem with the client, therapist or both, but may also be a sign of progress onto dealing with greater difficulties masked by the initial problem.
The impression given by many textbooks is that improvement is gradual and continuous. Clients progress smoothly from one treatment goal to the next until all issues have been resolved and they can be discharged from your caseload.
Many therapists experience a sinking feeling when a client who had been making progress reports no change (or worse, a deterioration) in mood or function (or both). Deterioration may indicate a lack of understanding. The client may have attempted to follow the therapist's advice without full comprehension. The client may therefore have been attempting a more difficult task than was intended (or even an impossible task). However clear the instructions, the practical implementation is the true test of their clarity.
Deterioration may indicate a breakdown of rapport. The client may have made a commitment with which they privately disagreed but did not feel able to tell the therapist. The therapist may have forced a commitment they felt to be in the client's best interests without being fully open to the client's reservations.
Deterioration may be due to events outwith the control of client or therapist. Someone who has experienced a significant life event (eg: a bereavement, an addition to the family, redundancy or promotion or even simple ill health) may have been too pressured by this to keep their mood diary or adhere to their exercise regime.
Deterioration may indicate that new information has come to light. Overcoming one difficulty may make permit a client to enter situations in which a second, possibly greater, difficulty is highlighted. This can be the most valuable of experiences, in that their previous experience of success in therapy can fortify the client for the next challenge.