I haven't worked on many concrete structures, recently. However, could you go into Visibility Graphics and change the transparency of your floors? See if that helps the beams to show up.

meh... strike that.
I just opened a new project and put in a test floor and test beams.
I saw dashed lines for my beams. In section, the beams and floor were joined without me having to join them to the slab.

So, we'll have to dig a little deeper to figure out what's going on.
What version of Revit are you using?
Are your concrete slab and your concrete beams the same material?
What is your visual style on your plan view?
What is your plan view Discipline? For me, Structural Discipline shows dashed lines for the beam; Coordination Discipline shows solid lines.

more testing, more playing...
I have to admit, I haven't been as active in Revit as I once was, so this trick I used years ago and (almost) forgot about...

I can recreate your issue when I'm in Discipline:Coordination and Visual Style: Hidden Line. (For the record, in my section view, my beams are already joined to my slab because all of my material is the same: Concrete, Cast-in-place gray; I did not have to use the join tool to get them to join.)

In your plan view, on your View Tab, Graphics Panel, click on the Show Hidden Lines tool; follow the prompts at the bottom of your screen. Choose your slab first, then choose the beams you want to see. It may be helpful to temporarily change your visual style to Wireframe so that you can see all the beams you want to show through your slab. Then you can change your view back to Hidden Line.

Thank you. I spoke to my CAD manger, he said the model we are working was acquired from some other company hence there are these sort of issues.

Revit 2017 version
Slab and beams are concrete (Same material)
Hidden line is the visual style
Discipline is structural and also show hidden line (by discipline)