I have an avocado plant I started from a pit, it’s around 1.5 years old. Around a month ago we put it in a bigger pot, because it stopped growing and we thought it needed more space. In these last days though it doesn’t look good. Its leaves look weak and lifeless. Any idea what the problem could be?
Drooping is a sign of 'wet feet', hence overwatering (see comment of @Evil Elf). If you water the plant too often, the soil can get too wet (even if the top layer seems fine). Try to feel finger deep before you water it again. Also use a pot with good drainage.
Too much watering causes the roots to rot, and causes deprivation of oxygen (roots use oxygen). Therefore it can not uptake enough water for enough pressure to hold up the leaves.
If the soil is not too wet it might also be a temperature problem, but first suspect is overwatering.