This is a familiar problem. You have to keep in mind that what might appear as an obvious joke to you, could appear completely serious to others. You notice that by you joking around, you have confused the barrier between seriousness and joking enough for others to be unable to tell the difference. As a reaction, they will just assume you to be joking.
Fixing this will take time. Make sure that whenever you are serious, and want to be taken seriously, you tell them.
Hey guys, I'm not joking around, I don't have time right now, I have work to do.
Seriously now, I have work to do.
At first they will just assume it to be a joke, but if you make sure that whenever you say "seriously now", you actually are serious, and whenever you are serious, you make it clear that you are, they should catch on soon enough.
This also goes for serious faces and such, make sure that you act serious if and only if you intend to be taken seriously.
I personally like to make puns and random jokes, and sometimes I think I am being obviously ironic, others are unable to tell and take me seriously. This is something to be aware of.
Lastly, know when it is appropriate to make jokes and when it isn't. Make sure to flip the switch whenever it isn't. Make sure that others see that you are not just about joking around, but that you are able to flip the switch. That should also allow them to recognise you being serious more easily.