Your potting soil has sat for too long and the organic matter in your blocks are putrefying, also known as anaerobic, and the breakdown of this material is robbing your plants of nitrogen and oxygen. Blocking Mix should never stink, use finished compost and your best gardening soil. Make your block mix fresh and use within 2 hours. Any left over material should be set aside and reworked back into fresh blocking mix by churning and mixing thoroughly with your hands or a paddle. This is known as actively aerated potting soil, coined a.a.p.s. Keep churning your mix until it smells like freshly plowed soil. Then make your blocks again. If your blocking mix is too far gone for a remix, perhaps it sat for weeks, chuck it in the compost pile and start again. Also, stay away from manure composts and questionable composts. Again, use finished, matured, aged, and blackened compost or worm castings that ate mostly vegetable wastes.