Empty SELECT result at simultaneous calls

Hello guys,

I have a problem with my web application and postgres. I have several servlets on a page which results in severeal
simultaneousdata base calls kind of:  

SELECT d.id, d.aa, d.ab, ts.ac, d.ad, d.af, d.ag, d.ah, d.ai, d.aj, d.ak, d.al, d.am, d.an, d.ao, d.ap, d.ar, d.as,
d.at,d.au, d.av, d.ax, d.ay, d.az, d.ba, d.bb, d.bc FROM c_depots d INNER JOIN c_aa ts ON d.bd_id=ts.id INNER JOIN
cx_users_depotscx ON cx.id_depots=d.id INNER JOIN c_users u ON cx.id_users=u.id WHERE d.id=13 

Even though I have "d.id 13" the SELECT result is empty. When I'm working with delays in the servlets, everything works
fine.The calling instance itself is realized as single instance: 

    public static DbManager getInstance() {
        if(instance == null) {
            instance = new DbManager();
        }
        return instance;
    }

So I'm wondering why there are such problems and how can I resolve them?

My posgres version is (still) 8.2.

Thanks.