"Not up to" generally refers to something that is not satisfactory or up to the standard. There are several synonyms for this phrase that can be used depending on the context. For instance, the terms "substandard," "inferior," "insufficient," and "inadequate" can be used interchangeably. Additionally, "below par," "not fit for purpose," "unsatisfactory," or "below the mark" are also suitable synonyms. In the same vein, you may also use terms like "lacking," "unsuitable," "unacceptable," "inferior quality," or "deficient" to describe something that is "not up to" the expected standard. Essentially, the choice of synonym to use depends on the degree to which the performance, quality, or condition falls below expectations.