Although I do not have first hand experience with psoriasis i have watch my husband deal with it for years. My husband firmly believes in using psoriasis medications for short periods of time only because anything else causes it to get worse. He uses Urea 40% cream and/or clobetasol propionate .05%. If he applies medication more than occasionally it seems to change the psoriasis and it no longer responds to the original dosage. It sounds like your psoriasis is acting in a similar way in response to the medications you are taking.
Best of luck.