Some people are naturally easily trusting. Others struggle. If you have a client who’s struggling, suggest that s/he give you a small thing to handle (take a small risk). Handle it superbly. Make sure s/he’s delighted. Do it again. Then again. Every time the risk/good outcome cycle repeats, trust builds. You should be able to have a reasonable amount of trust built with any client inside of 30 days. If you don’t, consider that one or both of you may have chosen unwisely.