Is a person who repeatedly vows to Allah to abandon masturbation and sin, and then breaks his vow, obligated to offer multiple expiations or a single expiation? Is Allah's withholding of success in marriage from him a sign that his repentance has not been accepted, and will Allah forgive him for repeatedly breaking his vow?

If the vow was merely an intention in the heart, then nothing is incumbent upon it, and no expiation is required; because an intention does not constitute a vow or an oath except by uttering it. One must repent to Allah and mend one's relationship with Him, strive in supplication for Allah to avert evil, repent from every wrong deed, seek help through fasting, keep company with righteous women, and adhere to remembrance and supplication to ask for a righteous husband. For a servant may be deprived of provision due to his sins, and obedience to Allah and piety are among the greatest means of obtaining provision.