"Ive had then for maybe 5 years or so, red/purple flesh has...

"Ive had then for maybe 5 years or so, red/purple flesh has flowered twice and given me 2 fruit. The white one produced 8 flowers last year but no fruit, it has a flower on it now that should open any day now."
Try giving them a dose of soluble Potassium Sulphate. A deficiency in that will stop fruit developing and it sweetens them.