Thanks much,Loris.
The details are as belows:
temperature:19 degrees, humidity: 68%
I use a synthetic brush. After coating I let it dry naturally in the dark for twenty minutes.Then go for exposure. The developer is sodium citrate. Citrate acid and EDTA as clearing agent.

I haven't tried other papers.The FO is not fresh though. I bought it from B&S about 5 months ago. How to judge if it dues to the freshness of FO? Also could it be the problem of UV lights? It's too cloudy these days to have it exposed in the sun.