HCCONH, HBHACONH ,CCCONH are used for side chain assignment, not the backbone (but can help when there are ambiguity, because of discrimination of amino acid by side chain spin system). If you don't have enough pic in your CACBNH experiment (and CACBCONH), perhaps try more sensitive experiment set like HNCA/HN(CA)CO (CA and CO for aa i) and HN(CO)CA/HNCO (CA and CO for aa i-1), since you also have two carbon resonance to link amino acid together. This strategy is usually used for protein with a mass beyond 20kDa... Have you all the pic in your 15N-HSQC? Have you an idea of secondary structure (by homology) ? What's the protein concentration ?