First it may just be that your Anchusa, like the forget-me-not that it is related to, has bloomed for the year and is ready to be cut back so it can possibly flower again. If yours has not yet bloomed than yellowing leaves are most often a sign of too much water or soil that does not drain well or the plant is suffering from too much heat and dry soil. If you had a hot spell then water well but if you have had rain try to increase the drainage by poking holes around the roots. Keep growing, Marianne Binetti