i recently changed my heater matrix as the last one was leaking and it absolutely stunk!!

but anyway, its been in about a month now i suppose and when the engines warm i put it on, its hot for about 5 secs then it goes luke-warm?? why is this? have i put the heater pipes on the wrong way around?
sorry to start a new topic on heaters..i just need my suspicions clarifying as i think it could be this....and iv got shizzle loads of coursework to do and havnt got all day looking in the past posts..

thanks
dave