Why does this approach fail?

Ignoring the fabric limitations to simplify things we can see that skirts and blouses have a profit of 2€/hour (pants are less). This tells us that at best with only 150 hours available every month your profit is indeed caped at 300€. This is why you have losses - the machines are too expensive.
Two additional remarks:
Are you sure that the time limit of 150 hours is a limit on the total time of all three machines and the cost of each machine doesn't depend on its usage?
Since the per hour profit of skirts and blouses is the same you could simply produce 75 blouses (150h and 140$m^2$ of fabric). This gives you 300€ - 150€ = 150€ profit since you are not using two of the three machines.