orchid phalaenopsis

Maybe someone can help me with this one. I bought a lovely healthy orchid about 4 months ago and it has bloomed consistently since then but the leaves are looking very bad. I have lost 2 leaves already and the remaining 3 are starting to yellow and shrivel. On the underside of one of the leaves is a sticky clear substance but I can see no pests. 

 

I have followed instructions about watering, sunlight, temperature and have recently repotted the orchid as it seemed to be bursting out of the pot with the roots very compacted.

 

Does anyone have any ideas about what I am doing wrong?

 

thanks

It's my guess that the orchid may be stressed which can be due to a draft or incorrect watering. Mist spraying every week helps. And some fertiliser too when it's growing.