Often the effectiveness of the velcro can be affected by bits of fluff and threads getting stuck in the teeth of the velcro. You can pick these out with a needle or pin and this should help greatly!
If this isn’t the case, it may be due to a build-up of detergent on your nappies; particularly if you’re using a powder this can build up amongst the Velcro and hinder the attachment.
Firstly, try carefully brushing the Velcro pile with a comb to comb out the detergent, and if that doesn’t help try putting the nappies on a full wash with no detergent, and rinse until you no longer see any bubbles during the rinse cycle.
Finally the last suggestion is that sometimes this can be due to a build-up of calcium on the tabs from washing, and a way to improve this is to very carefully try dipping the tabs in an acidic solution such as white vinegar or nail varnish remover, (make sure no part of the nappy goes in, just the tab!) It may bubble a bit, and when it’s stopped bubbling it should have all dissolved and hopefully will stick again.
If the problem is just that the velcro tabs won’t stick to the washing tabs any more during the laundry cycle you can simply close the velcro tabs to the main Velcro strip around the waist, or even to each other!