Some of this is from my brain other bits I have harvested from others, makes sense though, pretty much.
Writers block does actually exist but usually it's just laziness, apathy or you're not ' feeling it'. Some things you can do to address these three are:
Unplug the Internet.
Set a specific time to write from and to and stick to it - even if you just stare at the page.
Write a scene ahead and go back.
Likely there is a plot point/gag/scene you are not loving and so you loose interest. I can feel an itch sometimes with a line I know something is up but I can't place my finger on exactly what. You need to be able to identify what is causing this and remove the offending line(s) and pick it up again. Sometimes remove it rather trying to fix it will work.
Sitcom rules followed by me (most of the time):
Write, write, write! Do not stop, however many good ideas you have or have written, do not stop. Writing improves your skill and is money in the bank.
Don't send a script unless it's as good as you can possibly make it, better to not send it than send shit.
Treat a script like a CV, make the first pages grab the reader.
Story; a sitcom still needs a story; a beginning, middle and end otherwise known as trouble, muddle and triumph/failure.
Characters need to feel alive you to and the reader, make them real and they will write themselves into your script. Make people care about your characters. What are the stakes if your character fails in his/her tasks?
Develop a set of rules for your sitcom and stick to them (see below).
Break rules willingly. Pay attention to advice and rules given by others and break them deliberately not accidentally (see above).
If a line isn't moving the plot on, isn't setting up a gag or is a punch line - cut it!
Use Final Draft and format correctly.
A sitcom is supposed to be funny, is yours?
Escalate, escalate, escalate! As soon as you can and keep doing it.