why are my espresso shots so weak and bitter

Under-extraction is the culprit behind weak and bitter espresso shots. This can be caused by a few issues: grind size or tamping consistency. If you're grinding too fine, it's likely under-extracting; if you're not tamping well, it's also a problem. Check your grind and tamp the puck evenly. Try coarser grinds and press down firmly with the tamper to ensure good contact between the grounds and the portafilter basket.