Can you describe your method in a bit more detail? You don't want to form a gigaseal, just a weak seal, which can be difficult. You also typically need very large voltages.. ~10 V... outside the range of typical patch clamp amplifiers. Finally, to get complete visualization, I would double that Alexa concentration. You're going to want to wait a solid 20 minutes after electroporation to wait for the dendrites to fill, but they're never going to be very clear unless you're under a laser scanning scope of some kind.

Thanks! I'm using the Axoporator of Molecular Devices, -12 V pulse trains, 9 MOhm pipettes and a weak seal of approx. 20 MOhm. I'm waiting about 15 min before I image my cells under a laser scanning confocal. The electroporation procedure does not seem to be the problem, since I fill the soma immediately and parts of the dendrites and axon. I will try increasing my Alexa concentration and wait a bit longer... maybe that does the trick

Yeah, then sounds like waiting longer will be the trick. Even when you patch a cell, it can take up to 30 minutes to fill all the dendrites. Even longer for massive cells if your access isn't great.