Possible drug interaction - Amoxicillin

I wasn't sure where to post this question as its about drug interactions really.

I've been taking anitbiotics, Amoxicillin and Doxycycline (Not both at the same time) to treat recurrent sinusitis but they haven't got on top of the infection. Last Autumn/Fall I had five courses of antibiotics and knew that I still hadn't got rid of the infection. The infection came back last week and I had another course of Amoxicillin. In the past when I've taken these antibiotics they've given me stomach ache but that's not happening now. I'm wondering if the medication I'm taking is reducing the effectiveness of the antibiotics.

I take Lansoprazole for acid reflux. I've taken it for about 8 years with no problems. I have recently started taking Ivabradine for post viral Inappropriate Sinus Tachycardia. Would either of these drugs reduce the effectiveness of the antibiotics?

Any thoughts would be very much appreciated. Thank you.