You have the same scope I do. I have never had a a hard time picking up Jupiter. In fact you should be able to see the planet with binoculars.
Perhaps you are not targetting Jupiter, but are on a star. Jupiter should be the brightest point in the sky except for the moon. This time of year it is high overhead. Check your finder scope. Is it lined up? You should do this in the day time by pointing the finder scope on a radio tower or other object that is pretty far off. When you have that object in the cross hairs than look at it through the focus lense. Make adjustments with the focus screws and the micro adjustments till the object is in the middle of the cross hairs and in the middle of the focus lense.
If you are sure you have Jupiter, than what about other sites. Have you located any objects and noticed that they to are hard to bring in focus? (When you get a chance check out the moon.) If they are out of focus you might have a problem with the scope. Maybe it needs cullminated (The Mirror may need lined up). If has been dropped in shipping or handling that might do it in.
I would suggest that when you get your scope on an object use the lower magnification lense first. That would be the 20mm. Slowly turn the the focus till you get a clear image. Remember that other forces could contribute to a bad image such as the athmosphere, heat, street lights etc.
If you get a good image with the 20, than increase to the 12.5mm. Every time you change eps you will have to refocus. You might want to use the barlow with the 20 and even the 12.5, but you will not get a good focused image using the barlow with the 4mm. That is too much magnification. In fact when you use the 4mm alone you will have a much harger time getting the image in focus and keeping the image in view.
I have used the sr4mm on planets and got good views.
Finally, You can't be much newer at this than I am for I got my scope for Christmas. I think I can relate to you because I have recently went through some of the problems you maybe having.
By the way, I like my Tasco very much, though there will be some who will tell you different. To each his own.
I hope I have been of some help.