Reasons a pavlova comes out soft or weeps - remembering that most of them weep a little depending on the weatherhumidity -
Overbeaten whites. You should beat using a stand mixer on about 75% of max speed for a maximum of 8-10 minutes or you risk the foam structure collapsing and the pavlova weeping. Start adding sugar at softfirm peak stage about 1 min 30s, about one dining tablespoon per 20s or so, aiming to have it all in by 6-8 minutes. This is because the foam forms too many bonds between the protein strands, these tighten and squeeze out both air and water molecules, leaving too much free water in the mix.
Undissolved sugar. Use caster/fine granulated sugar, aim to have it all in by 8 mins beating. By 10 mins beating it should be dissolved so that you can't feel grains if you rub a bit of mix between your fingers.
Undercooked foam, unlikely if instructions have been followed.
Overcooking. Unlikely if instructions have been followed and oven temperature is accurate.
Humidity - this is the big one. If the outside crust of the pavlova isn't crisp, humidity is the likely culprit. If this happens, you can put it in a small room and run a dehumidifier, which helps a bit. Leaving uncovered in refrigerator for several hours can also help as long as anything else that might evaporate is covered. Basically if it's humid, cook something else.
Ideally use egg whites at least a couple of weeks old, they foam to a larger volume.
Avoid fat, so use a clean metal bowl and beaters, no plastic, no yolk contamination.
Oven preheated (my own personal failing)
Set timer to ten minutes. Start beating at about 75-80% of max speed on a stand mixer. You can use a handheld, but I don't know the times for that. Start adding sugar at softfirmish peak stage, about one and a half mins. I use a table dessert spoon or tablespoon and add a rounded spoonful every 15-20s or so, flowing it in around the close edge of the bowl to avoid too much weight in one place.
Add acid after a couple of spoonfuls of sugar.
Finish sugar by 8 mins beating. Continue to beat until sugar is dissolved - test by rubbing between fingers. If you have to beat longer than 10 mins you risk collapse and a bit of weeping.
Fold in cornflour (stabilises it) and vanilla or other liquid flavouring if using.
Pile onto baking paper on tray in a rough circle about 4" high and roughly smooth the top. Cook as directed. This should involve turning off the oven at end of cooking and leaving in a closed oven until cool.