Which PayPal gateway is this? If it's PayPal Payments Standard and the transaction in Blesta doesn't appear, make sure your IPN is enabled in your PayPal account, if it's disabled then PayPal won't make the request. https://docs.blesta.com/display/user/PayPal+Payments+Standard#PayPalPaymentsStandard-CommonIssues
If the transaction does appear, and the invoice is paid, the cron will activate the service next time it runs, make sure you've set up the cron job correctly and that it's regularly running and updating the last ran date time under Settings > System > Automation.
Blesta provisions paid pending services, so the invoice should be paid, and the service should be pending. If the service is in-review, then the order has not been approved, or was held for manual review due to your settings and/or fraud options.