Question on planting from seeds. I am told to wrapped the pot up with plastics. but for how long?

It's been about a week, i can see little seedlings growing out of the soil already. now i only loosely wrap the pot up with a huge translucent plastic bag. I read from the instructions that it is to keep the moisture in. I have tried seeds planting many times and failed. Most of the time, many many seedlings grows out but they just die in their original containers one by one after one or two months. Sometimes they grow quite big but died after i transfer them to separate pots. I use fertile packet soil for seeds growing. I keep the pot on the corridoor, away from direct sunlight, but it can get quite hot to 30 degress celsius. I live in tropical country Singapore, high temperature and humid air. Plants i grow are like petunia and sweet william, supposedly easy to grow. It's very depressing. Please help.

Question on planting from seeds. I am told to wrapped the pot up with plastics. but for how long?
the reason for wrapping is to create a hot house effect.try planting them again but this time leave them to the elements.do keep the soil moist but not wet.if your seedlings are getting to a certain stage then dying you could have a damping off disease.discard the potting mix.wash all pots and potting equipment in bleach and start again with new soil.you can identify damping off by looking near the base of the plant.if it looks very thin and black and the upper part of the plant looks normal then you have damping off.very common problem.