most women ovulate between days 10 and 14, and not day 20. by starting on day 17 or 18 of your cycle with the primolut, and by taking it for ten days, you should have your period as soon as you stop the pills, which would be between days 28-30. this would shorten your cycles to more normal lengths, and if you ovulate, that should occur at the normal time, allowing you to get pregnant. the primolut will not stop you from ovulating, unless you take it at the beginning of the cycle, which you are not doing.
it is possible that you don't ovulate every cycle at all, in which case the primolut will not make you ovulate at all, though it won't stop it either. if you don't ovulate on your own, you may need to use clomid
either together with the primolut, or by itself, in order to make you ovulate each cycle.
I hope this answers your question, please let me know if you still have any questions.