That sounds fairly normal sally. The bacteriia that convert nitrate to nitrate take longer to establish themselves. The fact you have nitrates of 50 shows that there is a good colony building up, but it has a bit to go yet.
if I remember rightly, my nitrate was about 90 when my cycle finished, which is why you do nearly a 100% water change when the cycle does finally finish.
Shouldnt be long though

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TROPICAL TANK-Elite Tank - 207litres
6 Harlequin Rasboras, 5 Emerald Eye Rasboras, 10 Cherry Barbs, 2 Bristlenose Plecs, 2 Panda Corys. 4 Dwarf Chain Loaches.
ViaAqua Tank AR620 97litres
1 red male betta, 1 BN