Things that might be affecting your Afterpay limit:
are there sufficient funds on your card (generally they look to see 25% of the order value available to spend)
length of time you have been using Afterpay (typically $500 for new customers)
amount you have to repay, it may help to pay some off (not a guarantee)
adding a credit card may increase your spending power (not a guarantee)
Please contact Afterpay for more information about your specific situation.