Nerrines not good in pots but great in garden

Hi all

My bright pink nerrines are full off flowers in the garden, but the ones in pots have hardly flowered.  I thought they might be planted at the wrong depth and searched for info, but it conflicts.  One site says the bulbs should be above the soil a bit, and another says the soil should cover the bulb.  Help please. (They are planted in gritty compost).