Part of this is an image and perception problem. For example, some folks in the media have somehow managed to present the Democrats as somehow not being a majority.
Of course, the majority of the problem is based in reality: the Democrats are divided and seem to be unwilling to get things done. Part of the division is due to the fact that some Democrats seem to be Democrats in name only (the Blue Dogs, for example). Part of the division is that the Democrats are also divided on the left. Given this lack of unity (especially when contrasted with the Republicans) it is rather challenging for the Democrats to get it together. Hence, while the Democrats have a majority it is a divided majority.
Another part of the challenge for the Democrats is that the Republicans are rather unified and seem dedicated to fighting a holding action. To be specific, the Republican strategy seems to be to block the Democrats until Americans get sick of the deadlock and vote the Republicans back into power (which will be rather ironic). Once the Republicans get back into power, they will no doubt chastise the Democrats if they don’t work with them. After all, when the Republicans are in power they (and their Fox friends) love to talk about how people need to support the President and do things for the good of the country.