ivabradine has stopped working?

started ivabradine (2.5mg twice a day, trial dose because i’m sensitive to medication and have had bad reactions in the past) on the 12th of december. it helped massively for maybe 2 weeks but end of last week and today i’m right back where i started, heart spiking into the 140s+ post prandial tachycardia is bad again, it’s as if nothing has changed. my doctor said to follow up in a month and hopefully i’ll go up to 10mg but has my body rejected the medicine? or is it just because it’s such a low dose? (i also can’t take beta blockers due to asthma so ivabradine is literally my only hope)

sorry if this didn’t make sense, i’m incredibly frustrated.