First, try with more 'coils', when you tie the taut-line hitch; every coil adds some friction.
An alternative, that I, myself, like a lot, since I find it easier to tie, is the Farrimond Friction Hitch. And again here, if your rope is slippery, wet or stiff, add some more loops/coils, and make sure you 'dress' the hitch so it's good and tight.
A major advantage of the Farrimond Friction Hitch (over the midshipman's/taut-line hitch) is that it can be done on a bight, whereas the midshipman's/taut-line can only be done when the rope end is available.
A Farrimond Friction Hitch, by default, is also a slipknot, so it's easier to untie than the taut-line hitch.
An addition to my answer:
Also note, that both of the above hitches 'only' work when there is no or little friction between the rope and the pole you are tying around. The loop around the pole must be able to slide, since the hitch really only bites down on the standing end, when there is a pull on the part of the loop that is opposite to the standing end.
Hard to explain with words, so here's a picture