I did SEM for my TAM model and found high cross loadings between PU and BI indicators. However, factor loadings on respective constructs are higher than cross loadings and Fornell-Larker criterion is OK.
We would need more detail on how you specified your SEM. But have you read this article?
Henseler, J. , Ringle, C.M. and Sarstedt, M. (2015), “A new criterion for assessing discriminant validity in variance-based structural equation modeling”, Journal of the Academy of Marketing Science , Vol. 43 No. 1, pp. 115-135.
They found that neither the Fornell-Larcker criterion nor cross-loadings assessment provides a good determination of discriminant validity in a variance-based SEM. They also offer an alternative to Fornell-Larcker.