Newbie cymbidium help - brown tips on leaves

[Lorrette]

Hi everyone,

I'm new to both orchids and this forum, so if i'm asking this in the wrong place please let me know!

I purchased some small cymbidium plants last October, which is the first time I've tried to grow orchids. On the whole I think they are doing OK with some of them currently growing new leaves. However, one of the orchids developed a brown area on the tip of one of the leaves, which over 2-3 days spread down the leaf until the leaf died and fell off. Now this has happened again on a different plant, with two of the leaves now starting to go brown at the tip. I have attached a couple of photos I took with my phone this morning - apologies for the quality.

The orchids are all housed in a miniature portable greenhouse, which is covered in 50% shadecloth. They are on a 3rd floor north-facing balcony, which does get direct sun during the day (hence the use of the shadecloth). I'm in Melbourne, Australia, so the temperatures regularly get above 30 degrees celcius in summer with the nights staying quite warm.

Since I am so new to orchids I was wondering if you could tell me what could be causing these brown leaves, what can be done to prevent it and if it's anything I need to worry about.

Thanks very much in advance

Lorrette.