You don’t say what kind of lumps they are or how big they are. If they are flux, try a new crucible and using less flux in the future.
If they are metal, it’s likely that you aren’t mixing the alloy well enough. Or you might be using old or contaminated metal.
If they are investment, it could be any number of causes, ranging from old investment, incorrect proportions of investment and water, temperature of the water and/investment isn’t right or it could be that you are burning out too fast.
Without more info as to the nature of the lumps it’s hard to say anything more helpful.