Have tried planting them in late fall when the can get root established without the stress of summer warm soils in combination with summer irrigation? They can be very susceptible from sudden death by fungal root attacks in warm moist soils. The can do very well with perfect drainage and being grown a bit "hard", and starting them in fall is better for long term success. Also, I find they can be quite useful as large container plants in cool full sun or bright dappled shade for a couple of years. Only the straight Mimulus aurantiacus has lived/bloomed for me for five-plus years, the hybrids have usually only lasted three years or less. So always take some cuttings of colors you like to perpetuate them.

Good advice Bahia!

It would help to know what conditions you're trying to grow the plant under ...

Mimulus aurantiacus likes sun and dry summers. I planted mine in February in a spot with an eastern exposure. I watered about once a week to get it and other plants established. It grew quickly to 3-feet tall, and has been full of flowers since May. I continue to water about once a week because there are other plants that need the moisture, but I think the Mimulus would do fine with less water.