I am writing to draw your attention to the technical problems that we have been experiencing with your software. We have called your office several times already, and I find it most unsatisfactory that we have not been notified of the said problem.
I would be grateful if you would offer me an explanation and assure me that this problem will not arise again.
If I do not hear from you, I regret to inform you that I will be obliged to find oth er softwares that are foolproof.