Which plants are you using? Not every plant works well in an enclosure. Some need more/less humidity, more/less airflow, some are too delicate to withstand getting jumped on by geckos. If you could tell us what you're using, we might be able to give more info. Fertilizer isn't usually a problem for a few months, but too much/too little light can cause problems in a matter of a few weeks.
If you aren't sure of the plant names, can you post pictures? Seeing just what they look like can also help pinpoint the problem.