Flower spike stopped growing

[MCSchrum]

I have a Wilsonara 'Tiger Brew' that started a flower spike a couple of months ago. The spike grew to about 8 inches, and then stopped growing. Near the top of the spike, there are a couple of small buds about 1/4 of an inch in diameter. These buds haven't grown any larger in weeks. This orchid is potted in a fine bark mixture, and it's sitting in a east-facing window. The temp gets into the low to mid 80s on hot days, and down into the 60s at night. I water it twice a weak, fertilize at 1/4 strength once a week, and try to maintain at least 50% humidity. The plant sits fairly close to the window. What could be causing this flower spike to stop?