There's a few possible reasons for it.
(1) The coders don't know about it. Often everyone just assumes they do know. And they don't. Maybe it's sprung up randomly in a place they weren't expecting, maybe it's very rare, but in any case, unless they've acknowledged it or mentioned it in some way, send them an email about it.
(2) It's not very high priority. Sometimes you're looking at a "well, I *could* fix this bug that annoys one person out of every thousand . . . or I could go fix that other bug that has the potential to bring down the entire site. Hmm, what to do, what to do." Sometimes it's not even that clear-cut - sometimes it's a choice between a minor bug and a major feature. Sometimes the feature wins. If the bug is causing serious problems for you, again, use whatever bug reporting system they have and report it.
(3) They're gathering data. This happens all the time - you know there's a bug, but it's impossible to reliably reproduce, so you add a ton of debug output and wait for it to happen again. A week later it happens again and you add a bunch more debug output. Sometimes bugs can survive for *months* just because nobody can figure out what exactly happens.
(4) They're actively working on it. Sometimes bugs end up being bizarre artifacts caused by the system design, and can be amazingly hard to squash entirely.
(5) They're merely incompetent. I'd like to pretend this never happens, but I'd be lying, some coders just suck.
What *doesn't* happen is a bunch of competent coders get together, say "hey look! There's a critical bug here that will take five minutes to fix! Let's go get pizza and slack off for a few days." And making catty comments about "I wonder if they're planning to fix ____" just doesn't help. Trust me - if it's major, and the coders are competent, they're probably hating its existence just as much as you do.
Now, if it's a minor bug, it's possible they're *not* planning to fix it. However, if it's irritating you sufficiently, it's no longer a minor bug - so email them and tell them. In general, coders' priorities are pretty subjective things, and if we don't think anyone cares about a bug it's likely to end up on the bottom of the pile. And our piles can get really, really, really big.
Give your coders good info and a little bit of time. Without it, we can't do our job.